def _bem_explain_surface(id_):
 _rev_dict = dict(((val, key) for (key, val) in _surf_dict.items()))
 return _rev_dict[id_]

def _bem_explain_surface ( id_ ) : _rev_dict = dict ( ( ( val , key ) for ( key , val ) in _surf_dict . items ( ) ) ) return _rev_dict [ id_ ]
return a string corresponding to the given surface id .