def deriv(self, x: str, ctype: ContentType) -> SchemaPattern:
        """Return derivative of the receiver."""
        return (Empty() if
                self.name == x and self._active(ctype)
                else NotAllowed())
def deriv ( self , x : str , ctype : ContentType ) -> SchemaPattern : return ( Empty ( ) if self . name == x and self . _active ( ctype ) else NotAllowed ( ) )
Return derivative of the receiver.