/// 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);}