func (mt *MerkleTester) join(a, b []byte) []byte {
	return sum(sha256.New(), append(append([]byte{1}, a...), b...))
}
join returns the sha256 hash of 0x01 || a || b.
package merkletree
import (
	"bytes"
	"crypto/rand"
	"crypto/sha256"
	"math/big"
	"strconv"
	"testing"
	"github.com/NebulousLabs/errors"
	"github.com/NebulousLabs/fastrand"
)
type MerkleTester struct {
	data [][]byte
	leaves [][]byte
	roots map[int][]byte
	proofSets map[int]map[int][][]byte
	*testing.T
}