// File: contracts/StrongNFTV2.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import './oz/contracts/GSN/Context.sol';
import './oz/contracts/access/Ownable.sol';
import './oz/contracts/access/AccessControl.sol';
import './oz/contracts/token/ERC1155/ERC1155.sol';
import './oz/contracts/token/ERC1155/ERC1155Burnable.sol';
import './oz/contracts/token/ERC1155/ERC1155Pausable.sol';

contract StrongNFTV2 is Context, AccessControl, Ownable, ERC1155Burnable, ERC1155Pausable {
  bytes32 public constant MINTER_ROLE = keccak256('MINTER_ROLE');
  bytes32 public constant PAUSER_ROLE = keccak256('PAUSER_ROLE');

  /**
   * @dev Grants `DEFAULT_ADMIN_ROLE`, `MINTER_ROLE`, and `PAUSER_ROLE` to the account that
   * deploys the contract.
   */
  constructor(string memory uri) public ERC1155(uri) {
    _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
    _setupRole(MINTER_ROLE, _msgSender());
    _setupRole(PAUSER_ROLE, _msgSender());
  }

  /**
   * @dev Creates `amount` new tokens for `to`, of token type `id`.
   *
   * See {ERC1155-_mint}.
   *
   * Requirements:
   *
   * - the caller must have the `MINTER_ROLE`.
   */
  function mint(
    address to,
    uint256 id,
    uint256 amount,
    bytes memory data
  ) public virtual {
    require(hasRole(MINTER_ROLE, _msgSender()), 'ERC1155: must have minter role to mint');

    _mint(to, id, amount, data);
  }

  /**
   * @dev xref:ROOT:erc1155.adoc#batch-operations[Batched] variant of {mint}.
   */
  function mintBatch(
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) public virtual {
    require(hasRole(MINTER_ROLE, _msgSender()), 'ERC1155: must have minter role to mint');

    _mintBatch(to, ids, amounts, data);
  }

  /**
   * @dev Pauses all token transfers.
   *
   * See {ERC1155Pausable} and {Pausable-_pause}.
   *
   * Requirements:
   *
   * - the caller must have the `PAUSER_ROLE`.
   */
  function pause() public virtual {
    require(hasRole(PAUSER_ROLE, _msgSender()), 'ERC1155: must have pauser role to pause');

    _pause();
  }

  /**
   * @dev Unpauses all token transfers.
   *
   * See {ERC1155Pausable} and {Pausable-_unpause}.
   *
   * Requirements:
   *
   * - the caller must have the `PAUSER_ROLE`.
   */
  function unpause() public virtual {
    require(hasRole(PAUSER_ROLE, _msgSender()), 'ERC1155: must have pauser role to unpause');

    _unpause();
  }

  function _beforeTokenTransfer(
    address operator,
    address from,
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) internal virtual override(ERC1155, ERC1155Pausable) {
    super._beforeTokenTransfer(operator, from, to, ids, amounts, data);
  }
}

// File: contracts/oz/contracts/GSN/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: contracts/oz/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../GSN/Context.sol";
/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(_owner == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

// File: contracts/oz/contracts/access/AccessControl.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../utils/EnumerableSet.sol";
import "../utils/Address.sol";
import "../GSN/Context.sol";

/**
 * @dev Contract module that allows children to implement role-based access
 * control mechanisms.
 *
 * Roles are referred to by their `bytes32` identifier. These should be exposed
 * in the external API and be unique. The best way to achieve this is by
 * using `public constant` hash digests:
 *
 * ```
 * bytes32 public constant MY_ROLE = keccak256("MY_ROLE");
 * ```
 *
 * Roles can be used to represent a set of permissions. To restrict access to a
 * function call, use {hasRole}:
 *
 * ```
 * function foo() public {
 *     require(hasRole(MY_ROLE, msg.sender));
 *     ...
 * }
 * ```
 *
 * Roles can be granted and revoked dynamically via the {grantRole} and
 * {revokeRole} functions. Each role has an associated admin role, and only
 * accounts that have a role's admin role can call {grantRole} and {revokeRole}.
 *
 * By default, the admin role for all roles is `DEFAULT_ADMIN_ROLE`, which means
 * that only accounts with this role will be able to grant or revoke other
 * roles. More complex role relationships can be created by using
 * {_setRoleAdmin}.
 *
 * WARNING: The `DEFAULT_ADMIN_ROLE` is also its own admin: it has permission to
 * grant and revoke this role. Extra precautions should be taken to secure
 * accounts that have been granted it.
 */
abstract contract AccessControl is Context {
    using EnumerableSet for EnumerableSet.AddressSet;
    using Address for address;

    struct RoleData {
        EnumerableSet.AddressSet members;
        bytes32 adminRole;
    }

    mapping (bytes32 => RoleData) private _roles;

    bytes32 public constant DEFAULT_ADMIN_ROLE = 0x00;

    /**
     * @dev Emitted when `newAdminRole` is set as ``role``'s admin role, replacing `previousAdminRole`
     *
     * `DEFAULT_ADMIN_ROLE` is the starting admin for all roles, despite
     * {RoleAdminChanged} not being emitted signaling this.
     *
     * _Available since v3.1._
     */
    event RoleAdminChanged(bytes32 indexed role, bytes32 indexed previousAdminRole, bytes32 indexed newAdminRole);

    /**
     * @dev Emitted when `account` is granted `role`.
     *
     * `sender` is the account that originated the contract call, an admin role
     * bearer except when using {_setupRole}.
     */
    event RoleGranted(bytes32 indexed role, address indexed account, address indexed sender);

    /**
     * @dev Emitted when `account` is revoked `role`.
     *
     * `sender` is the account that originated the contract call:
     *   - if using `revokeRole`, it is the admin role bearer
     *   - if using `renounceRole`, it is the role bearer (i.e. `account`)
     */
    event RoleRevoked(bytes32 indexed role, address indexed account, address indexed sender);

    /**
     * @dev Returns `true` if `account` has been granted `role`.
     */
    function hasRole(bytes32 role, address account) public view returns (bool) {
        return _roles[role].members.contains(account);
    }

    /**
     * @dev Returns the number of accounts that have `role`. Can be used
     * together with {getRoleMember} to enumerate all bearers of a role.
     */
    function getRoleMemberCount(bytes32 role) public view returns (uint256) {
        return _roles[role].members.length();
    }

    /**
     * @dev Returns one of the accounts that have `role`. `index` must be a
     * value between 0 and {getRoleMemberCount}, non-inclusive.
     *
     * Role bearers are not sorted in any particular way, and their ordering may
     * change at any point.
     *
     * WARNING: When using {getRoleMember} and {getRoleMemberCount}, make sure
     * you perform all queries on the same block. See the following
     * https://forum.openzeppelin.com/t/iterating-over-elements-on-enumerableset-in-openzeppelin-contracts/2296[forum post]
     * for more information.
     */
    function getRoleMember(bytes32 role, uint256 index) public view returns (address) {
        return _roles[role].members.at(index);
    }

    /**
     * @dev Returns the admin role that controls `role`. See {grantRole} and
     * {revokeRole}.
     *
     * To change a role's admin, use {_setRoleAdmin}.
     */
    function getRoleAdmin(bytes32 role) public view returns (bytes32) {
        return _roles[role].adminRole;
    }

    /**
     * @dev Grants `role` to `account`.
     *
     * If `account` had not been already granted `role`, emits a {RoleGranted}
     * event.
     *
     * Requirements:
     *
     * - the caller must have ``role``'s admin role.
     */
    function grantRole(bytes32 role, address account) public virtual {
        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to grant");

        _grantRole(role, account);
    }

    /**
     * @dev Revokes `role` from `account`.
     *
     * If `account` had been granted `role`, emits a {RoleRevoked} event.
     *
     * Requirements:
     *
     * - the caller must have ``role``'s admin role.
     */
    function revokeRole(bytes32 role, address account) public virtual {
        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to revoke");

        _revokeRole(role, account);
    }

    /**
     * @dev Revokes `role` from the calling account.
     *
     * Roles are often managed via {grantRole} and {revokeRole}: this function's
     * purpose is to provide a mechanism for accounts to lose their privileges
     * if they are compromised (such as when a trusted device is misplaced).
     *
     * If the calling account had been granted `role`, emits a {RoleRevoked}
     * event.
     *
     * Requirements:
     *
     * - the caller must be `account`.
     */
    function renounceRole(bytes32 role, address account) public virtual {
        require(account == _msgSender(), "AccessControl: can only renounce roles for self");

        _revokeRole(role, account);
    }

    /**
     * @dev Grants `role` to `account`.
     *
     * If `account` had not been already granted `role`, emits a {RoleGranted}
     * event. Note that unlike {grantRole}, this function doesn't perform any
     * checks on the calling account.
     *
     * [WARNING]
     * ====
     * This function should only be called from the constructor when setting
     * up the initial roles for the system.
     *
     * Using this function in any other way is effectively circumventing the admin
     * system imposed by {AccessControl}.
     * ====
     */
    function _setupRole(bytes32 role, address account) internal virtual {
        _grantRole(role, account);
    }

    /**
     * @dev Sets `adminRole` as ``role``'s admin role.
     *
     * Emits a {RoleAdminChanged} event.
     */
    function _setRoleAdmin(bytes32 role, bytes32 adminRole) internal virtual {
        emit RoleAdminChanged(role, _roles[role].adminRole, adminRole);
        _roles[role].adminRole = adminRole;
    }

    function _grantRole(bytes32 role, address account) private {
        if (_roles[role].members.add(account)) {
            emit RoleGranted(role, account, _msgSender());
        }
    }

    function _revokeRole(bytes32 role, address account) private {
        if (_roles[role].members.remove(account)) {
            emit RoleRevoked(role, account, _msgSender());
        }
    }
}

// File: contracts/oz/contracts/token/ERC1155/ERC1155.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import './IERC1155.sol';
import './IERC1155MetadataURI.sol';
import './IERC1155Receiver.sol';
import '../../GSN/Context.sol';
import '../../introspection/ERC165.sol';
import '../../math/SafeMath.sol';
import '../../utils/Address.sol';

/**
 *
 * @dev Implementation of the basic standard multi-token.
 * See https://eips.ethereum.org/EIPS/eip-1155
 * Originally based on code by Enjin: https://github.com/enjin/erc-1155
 *
 * _Available since v3.1._
 */
contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
  using SafeMath for uint256;
  using Address for address;

  // Mapping from token ID to account balances
  mapping(uint256 => mapping(address => uint256)) private _balances;

  // Mapping from account to operator approvals
  mapping(address => mapping(address => bool)) private _operatorApprovals;

  mapping(address => mapping(uint256 => uint256)) private _ownerIdIndex;

  mapping(address => uint256[]) private _ownerIds;

  // Used as the URI for all token types by relying on ID substitution, e.g. https://token-cdn-domain/{id}.json
  string private _uri;

  /*
   *     bytes4(keccak256('balanceOf(address,uint256)')) == 0x00fdd58e
   *     bytes4(keccak256('balanceOfBatch(address[],uint256[])')) == 0x4e1273f4
   *     bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465
   *     bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c5
   *     bytes4(keccak256('safeTransferFrom(address,address,uint256,uint256,bytes)')) == 0xf242432a
   *     bytes4(keccak256('safeBatchTransferFrom(address,address,uint256[],uint256[],bytes)')) == 0x2eb2c2d6
   *
   *     => 0x00fdd58e ^ 0x4e1273f4 ^ 0xa22cb465 ^
   *        0xe985e9c5 ^ 0xf242432a ^ 0x2eb2c2d6 == 0xd9b67a26
   */
  bytes4 private constant _INTERFACE_ID_ERC1155 = 0xd9b67a26;

  /*
   *     bytes4(keccak256('uri(uint256)')) == 0x0e89341c
   */
  bytes4 private constant _INTERFACE_ID_ERC1155_METADATA_URI = 0x0e89341c;

  /**
   * @dev See {_setURI}.
   */
  constructor(string memory uri) public {
    _setURI(uri);

    // register the supported interfaces to conform to ERC1155 via ERC165
    _registerInterface(_INTERFACE_ID_ERC1155);

    // register the supported interfaces to conform to ERC1155MetadataURI via ERC165
    _registerInterface(_INTERFACE_ID_ERC1155_METADATA_URI);
  }

  /**
   * @dev See {IERC1155MetadataURI-uri}.
   *
   * This implementation returns the same URI for *all* token types. It relies
   * on the token type ID substitution mechanism
   * https://eips.ethereum.org/EIPS/eip-1155#metadata[defined in the EIP].
   *
   * Clients calling this function must replace the `\{id\}` substring with the
   * actual token type ID.
   */
  function uri(uint256) external view override returns (string memory) {
    return _uri;
  }

  /**
   * @dev See {IERC1155-balanceOf}.
   *
   * Requirements:
   *
   * - `account` cannot be the zero address.
   */
  function balanceOf(address account, uint256 id) public view override returns (uint256) {
    require(account != address(0), 'ERC1155: balance query for the zero address');
    return _balances[id][account];
  }

  /**
   * @dev See {IERC1155-balanceOfBatch}.
   *
   * Requirements:
   *
   * - `accounts` and `ids` must have the same length.
   */
  function balanceOfBatch(address[] memory accounts, uint256[] memory ids)
    public
    view
    override
    returns (uint256[] memory)
  {
    require(accounts.length == ids.length, 'ERC1155: accounts and ids length mismatch');

    uint256[] memory batchBalances = new uint256[](accounts.length);

    for (uint256 i = 0; i < accounts.length; ++i) {
      require(accounts[i] != address(0), 'ERC1155: batch balance query for the zero address');
      batchBalances[i] = _balances[ids[i]][accounts[i]];
    }

    return batchBalances;
  }

  /**
   * @dev See {IERC1155-setApprovalForAll}.
   */
  function setApprovalForAll(address operator, bool approved) public virtual override {
    require(_msgSender() != operator, 'ERC1155: setting approval status for self');

    _operatorApprovals[_msgSender()][operator] = approved;
    emit ApprovalForAll(_msgSender(), operator, approved);
  }

  /**
   * @dev See {IERC1155-isApprovedForAll}.
   */
  function isApprovedForAll(address account, address operator) public view override returns (bool) {
    return _operatorApprovals[account][operator];
  }

  /**
   * @dev See {IERC1155-safeTransferFrom}.
   */
  function safeTransferFrom(
    address from,
    address to,
    uint256 id,
    uint256 amount,
    bytes memory data
  ) public virtual override {
    require(to != address(0), 'ERC1155: transfer to the zero address');
    require(from == _msgSender() || isApprovedForAll(from, _msgSender()), 'ERC1155: caller is not owner nor approved');

    address operator = _msgSender();

    _beforeTokenTransfer(operator, from, to, _asSingletonArray(id), _asSingletonArray(amount), data);

    _balances[id][from] = _balances[id][from].sub(amount, 'ERC1155: insufficient balance for transfer');
    if (_balances[id][from] == 0 && _ownerIdExists(from, id)) {
      _deleteOwnerId(from, id);
    }
    _balances[id][to] = _balances[id][to].add(amount);
    if (!_ownerIdExists(to, id)) {
      _addOwnerId(to, id);
    }

    emit TransferSingle(operator, from, to, id, amount);

    _doSafeTransferAcceptanceCheck(operator, from, to, id, amount, data);
  }

  /**
   * @dev See {IERC1155-safeBatchTransferFrom}.
   */
  function safeBatchTransferFrom(
    address from,
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) public virtual override {
    require(ids.length == amounts.length, 'ERC1155: ids and amounts length mismatch');
    require(to != address(0), 'ERC1155: transfer to the zero address');
    require(
      from == _msgSender() || isApprovedForAll(from, _msgSender()),
      'ERC1155: transfer caller is not owner nor approved'
    );

    address operator = _msgSender();

    _beforeTokenTransfer(operator, from, to, ids, amounts, data);

    for (uint256 i = 0; i < ids.length; ++i) {
      uint256 id = ids[i];
      uint256 amount = amounts[i];

      _balances[id][from] = _balances[id][from].sub(amount, 'ERC1155: insufficient balance for transfer');
      if (_balances[id][from] == 0 && _ownerIdExists(from, id)) {
        _deleteOwnerId(from, id);
      }
      _balances[id][to] = _balances[id][to].add(amount);
      if (!_ownerIdExists(to, id)) {
        _addOwnerId(to, id);
      }
    }

    emit TransferBatch(operator, from, to, ids, amounts);

    _doSafeBatchTransferAcceptanceCheck(operator, from, to, ids, amounts, data);
  }

  function getOwnerIds(address owner) public view returns (uint256[] memory) {
    return _ownerIds[owner];
  }

  function getOwnerIdIndex(address owner, uint256 id) public view returns (uint256) {
    return _ownerIdIndex[owner][id];
  }

  /**
   * @dev Sets a new URI for all token types, by relying on the token type ID
   * substitution mechanism
   * https://eips.ethereum.org/EIPS/eip-1155#metadata[defined in the EIP].
   *
   * By this mechanism, any occurrence of the `\{id\}` substring in either the
   * URI or any of the amounts in the JSON file at said URI will be replaced by
   * clients with the token type ID.
   *
   * For example, the `https://token-cdn-domain/\{id\}.json` URI would be
   * interpreted by clients as
   * `https://token-cdn-domain/000000000000000000000000000000000000000000000000000000000004cce0.json`
   * for token type ID 0x4cce0.
   *
   * See {uri}.
   *
   * Because these URIs cannot be meaningfully represented by the {URI} event,
   * this function emits no events.
   */
  function _setURI(string memory newuri) internal virtual {
    _uri = newuri;
  }

  /**
   * @dev Creates `amount` tokens of token type `id`, and assigns them to `account`.
   *
   * Emits a {TransferSingle} event.
   *
   * Requirements:
   *
   * - `account` cannot be the zero address.
   * - If `to` refers to a smart contract, it must implement {IERC1155Receiver-onERC1155Received} and return the
   * acceptance magic value.
   */
  function _mint(
    address account,
    uint256 id,
    uint256 amount,
    bytes memory data
  ) internal virtual {
    require(account != address(0), 'ERC1155: mint to the zero address');

    address operator = _msgSender();

    _beforeTokenTransfer(operator, address(0), account, _asSingletonArray(id), _asSingletonArray(amount), data);

    _balances[id][account] = _balances[id][account].add(amount);
    if (!_ownerIdExists(account, id)) {
      _addOwnerId(account, id);
    }
    emit TransferSingle(operator, address(0), account, id, amount);

    _doSafeTransferAcceptanceCheck(operator, address(0), account, id, amount, data);
  }

  /**
   * @dev xref:ROOT:erc1155.adoc#batch-operations[Batched] version of {_mint}.
   *
   * Requirements:
   *
   * - `ids` and `amounts` must have the same length.
   * - If `to` refers to a smart contract, it must implement {IERC1155Receiver-onERC1155BatchReceived} and return the
   * acceptance magic value.
   */
  function _mintBatch(
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) internal virtual {
    require(to != address(0), 'ERC1155: mint to the zero address');
    require(ids.length == amounts.length, 'ERC1155: ids and amounts length mismatch');

    address operator = _msgSender();

    _beforeTokenTransfer(operator, address(0), to, ids, amounts, data);

    for (uint256 i = 0; i < ids.length; i++) {
      _balances[ids[i]][to] = amounts[i].add(_balances[ids[i]][to]);
      if (!_ownerIdExists(to, ids[i])) {
        _addOwnerId(to, ids[i]);
      }
    }

    emit TransferBatch(operator, address(0), to, ids, amounts);

    _doSafeBatchTransferAcceptanceCheck(operator, address(0), to, ids, amounts, data);
  }

  /**
   * @dev Destroys `amount` tokens of token type `id` from `account`
   *
   * Requirements:
   *
   * - `account` cannot be the zero address.
   * - `account` must have at least `amount` tokens of token type `id`.
   */
  function _burn(
    address account,
    uint256 id,
    uint256 amount
  ) internal virtual {
    require(account != address(0), 'ERC1155: burn from the zero address');

    address operator = _msgSender();

    _beforeTokenTransfer(operator, account, address(0), _asSingletonArray(id), _asSingletonArray(amount), '');

    _balances[id][account] = _balances[id][account].sub(amount, 'ERC1155: burn amount exceeds balance');
    if (_balances[id][account] == 0 && _ownerIdExists(account, id)) {
      _deleteOwnerId(account, id);
    }
    emit TransferSingle(operator, account, address(0), id, amount);
  }

  /**
   * @dev xref:ROOT:erc1155.adoc#batch-operations[Batched] version of {_burn}.
   *
   * Requirements:
   *
   * - `ids` and `amounts` must have the same length.
   */
  function _burnBatch(
    address account,
    uint256[] memory ids,
    uint256[] memory amounts
  ) internal virtual {
    require(account != address(0), 'ERC1155: burn from the zero address');
    require(ids.length == amounts.length, 'ERC1155: ids and amounts length mismatch');

    address operator = _msgSender();

    _beforeTokenTransfer(operator, account, address(0), ids, amounts, '');

    for (uint256 i = 0; i < ids.length; i++) {
      _balances[ids[i]][account] = _balances[ids[i]][account].sub(amounts[i], 'ERC1155: burn amount exceeds balance');
      if (_balances[ids[i]][account] == 0 && _ownerIdExists(account, ids[i])) {
        _deleteOwnerId(account, ids[i]);
      }
    }

    emit TransferBatch(operator, account, address(0), ids, amounts);
  }

  /**
   * @dev Hook that is called before any token transfer. This includes minting
   * and burning, as well as batched variants.
   *
   * The same hook is called on both single and batched variants. For single
   * transfers, the length of the `id` and `amount` arrays will be 1.
   *
   * Calling conditions (for each `id` and `amount` pair):
   *
   * - When `from` and `to` are both non-zero, `amount` of ``from``'s tokens
   * of token type `id` will be  transferred to `to`.
   * - When `from` is zero, `amount` tokens of token type `id` will be minted
   * for `to`.
   * - when `to` is zero, `amount` of ``from``'s tokens of token type `id`
   * will be burned.
   * - `from` and `to` are never both zero.
   * - `ids` and `amounts` have the same, non-zero length.
   *
   * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
   */
  function _beforeTokenTransfer(
    address operator,
    address from,
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) internal virtual {}

  function _doSafeTransferAcceptanceCheck(
    address operator,
    address from,
    address to,
    uint256 id,
    uint256 amount,
    bytes memory data
  ) private {
    if (to.isContract()) {
      try IERC1155Receiver(to).onERC1155Received(operator, from, id, amount, data) returns (bytes4 response) {
        if (response != IERC1155Receiver(to).onERC1155Received.selector) {
          revert('ERC1155: ERC1155Receiver rejected tokens');
        }
      } catch Error(string memory reason) {
        revert(reason);
      } catch {
        revert('ERC1155: transfer to non ERC1155Receiver implementer');
      }
    }
  }

  function _doSafeBatchTransferAcceptanceCheck(
    address operator,
    address from,
    address to,
    uint256[] memory ids,
    uint256[] memory amounts,
    bytes memory data
  ) private {
    if (to.isContract()) {
      try IERC1155Receiver(to).onERC1155BatchReceived(operator, from, ids, amounts, data) returns (bytes4 response) {
        if (response != IERC1155Receiver(to).onERC1155BatchReceived.selector) {
          revert('ERC1155: ERC1155Receiver rejected tokens');
        }
      } catch Error(string memory reason) {
        revert(reason);
      } catch {
        revert('ERC1155: transfer to non ERC1155Receiver implementer');
      }
    }
  }

  function _asSingletonArray(uint256 element) private pure returns (uint256[] memory) {
    uint256[] memory array = new uint256[](1);
    array[0] = element;

    return array;
  }

  function _deleteOwnerId(address owner, uint256 id) internal {
    uint256 lastIndex = _ownerIds[owner].length.sub(1);
    uint256 lastId = _ownerIds[owner][lastIndex];
    if (id == lastId) {
      _ownerIdIndex[owner][id] = 0;
      _ownerIds[owner].pop();
    } else {
      uint256 indexOfId = _ownerIdIndex[owner][id];
      _ownerIdIndex[owner][id] = 0;

      _ownerIds[owner][indexOfId] = lastId;
      _ownerIdIndex[owner][lastId] = indexOfId;
      _ownerIds[owner].pop();
    }
  }

  function _addOwnerId(address owner, uint256 id) internal {
    uint256 len = _ownerIds[owner].length;
    _ownerIdIndex[owner][id] = len;
    _ownerIds[owner].push(id);
  }

  function _ownerIdExists(address owner, uint256 id) internal view returns (bool) {
    if (_ownerIds[owner].length == 0) return false;
    uint256 index = _ownerIdIndex[owner][id];
    return id == _ownerIds[owner][index];
  }
}

// File: contracts/oz/contracts/token/ERC1155/ERC1155Burnable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "./ERC1155.sol";

/**
 * @dev Extension of {ERC1155} that allows token holders to destroy both their
 * own tokens and those that they have been approved to use.
 *
 * _Available since v3.1._
 */
abstract contract ERC1155Burnable is ERC1155 {
    function burn(address account, uint256 id, uint256 value) public virtual {
        require(
            account == _msgSender() || isApprovedForAll(account, _msgSender()),
            "ERC1155: caller is not owner nor approved"
        );

        _burn(account, id, value);
    }

    function burnBatch(address account, uint256[] memory ids, uint256[] memory values) public virtual {
        require(
            account == _msgSender() || isApprovedForAll(account, _msgSender()),
            "ERC1155: caller is not owner nor approved"
        );

        _burnBatch(account, ids, values);
    }
}

// File: contracts/oz/contracts/token/ERC1155/ERC1155Pausable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "./ERC1155.sol";
import "../../utils/Pausable.sol";

/**
 * @dev ERC1155 token with pausable token transfers, minting and burning.
 *
 * Useful for scenarios such as preventing trades until the end of an evaluation
 * period, or having an emergency switch for freezing all token transfers in the
 * event of a large bug.
 *
 * _Available since v3.1._
 */
abstract contract ERC1155Pausable is ERC1155, Pausable {
    /**
     * @dev See {ERC1155-_beforeTokenTransfer}.
     *
     * Requirements:
     *
     * - the contract must not be paused.
     */
    function _beforeTokenTransfer(
        address operator,
        address from,
        address to,
        uint256[] memory ids,
        uint256[] memory amounts,
        bytes memory data
    )
        internal virtual override
    {
        super._beforeTokenTransfer(operator, from, to, ids, amounts, data);

        require(!paused(), "ERC1155Pausable: token transfer while paused");
    }
}

// File: contracts/oz/contracts/utils/EnumerableSet.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Library for managing
 * https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets] of primitive
 * types.
 *
 * Sets have the following properties:
 *
 * - Elements are added, removed, and checked for existence in constant time
 * (O(1)).
 * - Elements are enumerated in O(n). No guarantees are made on the ordering.
 *
 * ```
 * contract Example {
 *     // Add the library methods
 *     using EnumerableSet for EnumerableSet.AddressSet;
 *
 *     // Declare a set state variable
 *     EnumerableSet.AddressSet private mySet;
 * }
 * ```
 *
 * As of v3.0.0, only sets of type `address` (`AddressSet`) and `uint256`
 * (`UintSet`) are supported.
 */
library EnumerableSet {
    // To implement this library for multiple types with as little code
    // repetition as possible, we write it in terms of a generic Set type with
    // bytes32 values.
    // The Set implementation uses private functions, and user-facing
    // implementations (such as AddressSet) are just wrappers around the
    // underlying Set.
    // This means that we can only create new EnumerableSets for types that fit
    // in bytes32.

    struct Set {
        // Storage of set values
        bytes32[] _values;

        // Position of the value in the `values` array, plus 1 because index 0
        // means a value is not in the set.
        mapping (bytes32 => uint256) _indexes;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function _add(Set storage set, bytes32 value) private returns (bool) {
        if (!_contains(set, value)) {
            set._values.push(value);
            // The value is stored at length-1, but we add 1 to all indexes
            // and use 0 as a sentinel value
            set._indexes[value] = set._values.length;
            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function _remove(Set storage set, bytes32 value) private returns (bool) {
        // We read and store the value's index to prevent multiple reads from the same storage slot
        uint256 valueIndex = set._indexes[value];

        if (valueIndex != 0) { // Equivalent to contains(set, value)
            // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
            // the array, and then remove the last element (sometimes called as 'swap and pop').
            // This modifies the order of the array, as noted in {at}.

            uint256 toDeleteIndex = valueIndex - 1;
            uint256 lastIndex = set._values.length - 1;

            // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
            // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.

            bytes32 lastvalue = set._values[lastIndex];

            // Move the last value to the index where the value to delete is
            set._values[toDeleteIndex] = lastvalue;
            // Update the index for the moved value
            set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based

            // Delete the slot where the moved value was stored
            set._values.pop();

            // Delete the index for the deleted slot
            delete set._indexes[value];

            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function _contains(Set storage set, bytes32 value) private view returns (bool) {
        return set._indexes[value] != 0;
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function _length(Set storage set) private view returns (uint256) {
        return set._values.length;
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function _at(Set storage set, uint256 index) private view returns (bytes32) {
        require(set._values.length > index, "EnumerableSet: index out of bounds");
        return set._values[index];
    }

    // AddressSet

    struct AddressSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(AddressSet storage set, address value) internal returns (bool) {
        return _add(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(AddressSet storage set, address value) internal returns (bool) {
        return _remove(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(AddressSet storage set, address value) internal view returns (bool) {
        return _contains(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns the number of values in the set. O(1).
     */
    function length(AddressSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(AddressSet storage set, uint256 index) internal view returns (address) {
        return address(uint256(_at(set._inner, index)));
    }


    // UintSet

    struct UintSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(UintSet storage set, uint256 value) internal returns (bool) {
        return _add(set._inner, bytes32(value));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        return _remove(set._inner, bytes32(value));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return _contains(set._inner, bytes32(value));
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function length(UintSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        return uint256(_at(set._inner, index));
    }
}

// File: contracts/oz/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies in extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return _functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        return _functionCallWithValue(target, data, value, errorMessage);
    }

    function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: weiValue }(data);
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: contracts/oz/contracts/token/ERC1155/IERC1155.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

import "../../introspection/IERC165.sol";

/**
 * @dev Required interface of an ERC1155 compliant contract, as defined in the
 * https://eips.ethereum.org/EIPS/eip-1155[EIP].
 *
 * _Available since v3.1._
 */
interface IERC1155 is IERC165 {
    /**
     * @dev Emitted when `value` tokens of token type `id` are transferred from `from` to `to` by `operator`.
     */
    event TransferSingle(address indexed operator, address indexed from, address indexed to, uint256 id, uint256 value);

    /**
     * @dev Equivalent to multiple {TransferSingle} events, where `operator`, `from` and `to` are the same for all
     * transfers.
     */
    event TransferBatch(address indexed operator, address indexed from, address indexed to, uint256[] ids, uint256[] values);

    /**
     * @dev Emitted when `account` grants or revokes permission to `operator` to transfer their tokens, according to
     * `approved`.
     */
    event ApprovalForAll(address indexed account, address indexed operator, bool approved);

    /**
     * @dev Emitted when the URI for token type `id` changes to `value`, if it is a non-programmatic URI.
     *
     * If an {URI} event was emitted for `id`, the standard
     * https://eips.ethereum.org/EIPS/eip-1155#metadata-extensions[guarantees] that `value` will equal the value
     * returned by {IERC1155MetadataURI-uri}.
     */
    event URI(string value, uint256 indexed id);

    /**
     * @dev Returns the amount of tokens of token type `id` owned by `account`.
     *
     * Requirements:
     *
     * - `account` cannot be the zero address.
     */
    function balanceOf(address account, uint256 id) external view returns (uint256);

    /**
     * @dev xref:ROOT:erc1155.adoc#batch-operations[Batched] version of {balanceOf}.
     *
     * Requirements:
     *
     * - `accounts` and `ids` must have the same length.
     */
    function balanceOfBatch(address[] calldata accounts, uint256[] calldata ids) external view returns (uint256[] memory);

    /**
     * @dev Grants or revokes permission to `operator` to transfer the caller's tokens, according to `approved`,
     *
     * Emits an {ApprovalForAll} event.
     *
     * Requirements:
     *
     * - `operator` cannot be the caller.
     */
    function setApprovalForAll(address operator, bool approved) external;

    /**
     * @dev Returns true if `operator` is approved to transfer ``account``'s tokens.
     *
     * See {setApprovalForAll}.
     */
    function isApprovedForAll(address account, address operator) external view returns (bool);

    /**
     * @dev Transfers `amount` tokens of token type `id` from `from` to `to`.
     *
     * Emits a {TransferSingle} event.
     *
     * Requirements:
     *
     * - `to` cannot be the zero address.
     * - If the caller is not `from`, it must be have been approved to spend ``from``'s tokens via {setApprovalForAll}.
     * - `from` must have a balance of tokens of type `id` of at least `amount`.
     * - If `to` refers to a smart contract, it must implement {IERC1155Receiver-onERC1155Received} and return the
     * acceptance magic value.
     */
    function safeTransferFrom(address from, address to, uint256 id, uint256 amount, bytes calldata data) external;

    /**
     * @dev xref:ROOT:erc1155.adoc#batch-operations[Batched] version of {safeTransferFrom}.
     *
     * Emits a {TransferBatch} event.
     *
     * Requirements:
     *
     * - `ids` and `amounts` must have the same length.
     * - If `to` refers to a smart contract, it must implement {IERC1155Receiver-onERC1155BatchReceived} and return the
     * acceptance magic value.
     */
    function safeBatchTransferFrom(address from, address to, uint256[] calldata ids, uint256[] calldata amounts, bytes calldata data) external;
}

// File: contracts/oz/contracts/token/ERC1155/IERC1155MetadataURI.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

import "./IERC1155.sol";

/**
 * @dev Interface of the optional ERC1155MetadataExtension interface, as defined
 * in the https://eips.ethereum.org/EIPS/eip-1155#metadata-extensions[EIP].
 *
 * _Available since v3.1._
 */
interface IERC1155MetadataURI is IERC1155 {
    /**
     * @dev Returns the URI for token type `id`.
     *
     * If the `\{id\}` substring is present in the URI, it must be replaced by
     * clients with the actual token type ID.
     */
    function uri(uint256 id) external view returns (string memory);
}

// File: contracts/oz/contracts/token/ERC1155/IERC1155Receiver.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../../introspection/IERC165.sol";

/**
 * _Available since v3.1._
 */
interface IERC1155Receiver is IERC165 {

    /**
        @dev Handles the receipt of a single ERC1155 token type. This function is
        called at the end of a `safeTransferFrom` after the balance has been updated.
        To accept the transfer, this must return
        `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))`
        (i.e. 0xf23a6e61, or its own function selector).
        @param operator The address which initiated the transfer (i.e. msg.sender)
        @param from The address which previously owned the token
        @param id The ID of the token being transferred
        @param value The amount of tokens being transferred
        @param data Additional data with no specified format
        @return `bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"))` if transfer is allowed
    */
    function onERC1155Received(
        address operator,
        address from,
        uint256 id,
        uint256 value,
        bytes calldata data
    )
        external
        returns(bytes4);

    /**
        @dev Handles the receipt of a multiple ERC1155 token types. This function
        is called at the end of a `safeBatchTransferFrom` after the balances have
        been updated. To accept the transfer(s), this must return
        `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))`
        (i.e. 0xbc197c81, or its own function selector).
        @param operator The address which initiated the batch transfer (i.e. msg.sender)
        @param from The address which previously owned the token
        @param ids An array containing ids of each token being transferred (order and length must match values array)
        @param values An array containing amounts of each token being transferred (order and length must match ids array)
        @param data Additional data with no specified format
        @return `bytes4(keccak256("onERC1155BatchReceived(address,address,uint256[],uint256[],bytes)"))` if transfer is allowed
    */
    function onERC1155BatchReceived(
        address operator,
        address from,
        uint256[] calldata ids,
        uint256[] calldata values,
        bytes calldata data
    )
        external
        returns(bytes4);
}

// File: contracts/oz/contracts/introspection/ERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "./IERC165.sol";

/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts may inherit from this and call {_registerInterface} to declare
 * their support of an interface.
 */
contract ERC165 is IERC165 {
    /*
     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
     */
    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

    /**
     * @dev Mapping of interface ids to whether or not it's supported.
     */
    mapping(bytes4 => bool) private _supportedInterfaces;

    constructor () internal {
        // Derived contracts need only register support for their own interfaces,
        // we register support for ERC165 itself here
        _registerInterface(_INTERFACE_ID_ERC165);
    }

    /**
     * @dev See {IERC165-supportsInterface}.
     *
     * Time complexity O(1), guaranteed to always use less than 30 000 gas.
     */
    function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
        return _supportedInterfaces[interfaceId];
    }

    /**
     * @dev Registers the contract as an implementer of the interface defined by
     * `interfaceId`. Support of the actual ERC165 interface is automatic and
     * registering its interface id is not required.
     *
     * See {IERC165-supportsInterface}.
     *
     * Requirements:
     *
     * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
     */
    function _registerInterface(bytes4 interfaceId) internal virtual {
        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
        _supportedInterfaces[interfaceId] = true;
    }
}

// File: contracts/oz/contracts/math/SafeMath.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: contracts/oz/contracts/introspection/IERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Interface of the ERC165 standard, as defined in the
 * https://eips.ethereum.org/EIPS/eip-165[EIP].
 *
 * Implementers can declare support of contract interfaces, which can then be
 * queried by others ({ERC165Checker}).
 *
 * For an implementation, see {ERC165}.
 */
interface IERC165 {
    /**
     * @dev Returns true if this contract implements the interface defined by
     * `interfaceId`. See the corresponding
     * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
     * to learn more about how these ids are created.
     *
     * This function call must use less than 30 000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

// File: contracts/oz/contracts/utils/Pausable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../GSN/Context.sol";

/**
 * @dev Contract module which allows children to implement an emergency stop
 * mechanism that can be triggered by an authorized account.
 *
 * This module is used through inheritance. It will make available the
 * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
 * the functions of your contract. Note that they will not be pausable by
 * simply including this module, only once the modifiers are put in place.
 */
contract Pausable is Context {
    /**
     * @dev Emitted when the pause is triggered by `account`.
     */
    event Paused(address account);

    /**
     * @dev Emitted when the pause is lifted by `account`.
     */
    event Unpaused(address account);

    bool private _paused;

    /**
     * @dev Initializes the contract in unpaused state.
     */
    constructor () internal {
        _paused = false;
    }

    /**
     * @dev Returns true if the contract is paused, and false otherwise.
     */
    function paused() public view returns (bool) {
        return _paused;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        require(!_paused, "Pausable: paused");
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        require(_paused, "Pausable: not paused");
        _;
    }

    /**
     * @dev Triggers stopped state.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    function _pause() internal virtual whenNotPaused {
        _paused = true;
        emit Paused(_msgSender());
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        _paused = false;
        emit Unpaused(_msgSender());
    }
}