def remove_observer(self, callback):
if callback not in self._observers:<EOL><INDENT>raise ValueError('<STR_LIT>'<EOL>.format(callback, self))<EOL><DEDENT>self._observers.remove(callback)<EOL>
Remove an observer from this event.

        Args:
            callback: A function or coroutine callback to remove from this
                event.

        Raises:
            ValueError: If the callback is not an observer of this event.