Remove from unordered multiset the value_type objects starting at the specified first position up to, but not including the specified last position, and return last. This method invalidates only iterators and references to the removed element and previously saved values of the end() iterator, and preserves the relative order of the elements not removed. The behavior is undefined unless first and last either refer to elements in this unordered multiset or are the end iterator, and the first position is at or before the last position in the sequence provided by this container.