/// Returns the pair (lower_bound(x), upper_bound(x)).
virtual std::pair<const_iterator,const_iterator> equal_range(const key_type& x) const
{return _base.equal_range(x);}
/// Returns the pair (lower_bound(x), upper_bound(x)).
virtual std::pair<iterator, iterator> equal_range(const key_type& x)
{return _base.equal_range(x);}