explicit multiset(const Compare& comp = Compare(),
const Allocator& = Allocator());
Effects: Constructs an empty set using the specified comparison object and allocator.
Complexity: Constant.
template <class InputIterator>
multiset(InputIterator first, last,
const Compare& comp = Compare(), const Allocator& = Allocator());
Requires: If the iterator's dereference operator returns an lvalue or a const rvalue, then Key shall be CopyConstructible.
Effects: Constructs an empty multiset using the specified comparison object and allocator, and inserts elements from the range [first,last).
Complexity: Linear in N if the range [first,last) is already sorted using comp and otherwise N logN, where N is last - first.