/// Same as find above but returns a const_iterator.
virtual const_iterator find(const key_type& x) const
{return _base.find(x);}
/// Searches the map for a pair with the key value x and returns an iterator to that pair if it is found. If such a pair is not found the value end() is returned.
virtual iterator find(const key_type& x)
{return _base.find(x);}