def __add__(self, inserter):
        """Concatenate two inserters."""
        for key, values in self._to_insert.items():
            values.extend([_ for _ in inserter._to_insert[key]
                           if _ not in values])
        return self
def __add__ ( self , inserter ) : for key , values in self . _to_insert . items ( ) : values . extend ( [ _ for _ in inserter . _to_insert [ key ] if _ not in values ] ) return self
Concatenate two inserters.