=====Priority queue constructors=====
Syntax:

<code>
    #include <queue>
    explicit priority_queue( const Compare& cmp = Compare(), 
                             const Container& c = Container() );

    priority_queue( input_iterator start, 
                    input_iterator end, 
                    const Compare& comp = Compare(), 
                    const Container& c = Container() );
</code>

Priority queues can be constructed with an optional compare function ''cmp'' and an
optional container ''c''. If ''start'' and ''end'' are specified, the priority queue will
be constructed with the elements between ''start'' and ''end''.