Obtains a cancellation function that removes elts.

        The returned cancellation function returns ``True`` if all elements
        was removed successfully from the _in_deque, and false if it was not.

def _canceller_for(self, elts, event):
        """Obtains a cancellation function that removes elts.

        The returned cancellation function returns ``True`` if all elements
        was removed successfully from the _in_deque, and false if it was not.
        """
        def canceller():
            """Cancels submission of ``elts`` as part of this bundle.

            Returns:
               bool: ``False`` if any of elements had already been sent,
               otherwise ``True``.
            """
            try:
                self._event_deque.remove(event)
                self._in_deque.remove(elts)
                return True
            except ValueError:
                return False

        return canceller