mapped_type& at(const key_type& _Keyval)
{   // find element matching _Keyval
iterator _Where = _Mybase::lower_bound(_Keyval);
if (_Where == _Mybase::end()
    || _Mybase::_Getcomp()(_Keyval, _Mybase::_Key(_Where._Mynode())))
    _Xout_of_range("invalid map<K, T> key");
return (_Where->second);
}