A flat_multiset is a container adaptor
that provides an associative container interface
that supports equivalent keys
(i.e., possibly containing multiple copies of the same key value) and
provides for fast retrieval of the keys themselves.
the time complexity of the operations
that insert or erase a single element from the
set is linear,
including the ones that take an insertion position iterator.
Descriptions are provided here only for operations on flat_multiset
that are not described in one of the general sections or
for operations where there is additional semantic information.
The effect of calling a constructor or member function
that takes a sorted_equivalent_t argument with a range
that is not sorted with respect to key_comp() is undefined.