    *  @tparam  _Alloc  Allocator type, defaults to allocator<_Key>.    *    *  Meets the requirements of a <a href="tables.html#00">container</a>, and    *  <a href="tables.html#xx">unordered associative container</a>    *    *  Base is _Hashtable, dispatched at compile time via template    *  alias __u