An unordered_multimap is an unordered associative container
that supports equivalent keys (an instance of unordered_multimap may contain
multiple copies of each key value) and that associates values of
another type mapped_type with the keys.
The unordered_multimap class
supports forward iterators.
It provides the operations described in the
preceding requirements table for equivalent keys; that is, an unordered_multimap
supports the a_eq operations in that table, not the a_uniq operations.
For an unordered_multimap<Key, T> the key_type is Key,
the mapped_type is T,
and the value_type is pair<const Key, T>.
Subclause [unord.multimap] only describes operations on unordered_multimap
that are not described in one of the requirement tables, or for which
there is additional semantic information.