Defined in header <algorithm> template< class ForwardIt, class T >
std::pair<ForwardIt,ForwardIt>
equal_range( ForwardIt first, ForwardIt last,
const T& value ); (1) template< class ForwardIt, class T, class Compare >
std::pair<ForwardIt,ForwardIt>
equal_range( ForwardIt first, ForwardIt last,
const T& value, Compare comp ); (2)
Returns a range containing all elements equivalent to value in th