Syntax:
#include <set> multiset(); multiset( const multiset& c ); ~multiset();
Every multiset has a default constructor, copy constructor, and destructor.
The default constructor takes no arguments, creates a new instance of that multiset, and runs in constant time. The default copy constructor runs in linear time and can be used to create a new multiset that is a copy of the given multiset c.
The default destructor is called when the multiset should be destroyed.