Cancel an active coroutine and remove it from the schedule.

        Args:
            identifier (typing.Any): The identifier returned from add.
            exc_type (typing.Optional[type]): The exception type to throw into
                the coroutine on cancel. No exception is thrown if nothing is
                given. Instead the coroutine is no longer processed.

        Returns:
            bool: True if the coroutine is cancelled. False if the identifier
                is invalid or if the coroutine is complete.

def cancel(
            self,
            identifier: typing.Any,
            exc_type: typing.Optional[type]=None,
    ) -> bool:
        """Cancel an active coroutine and remove it from the schedule.

        Args:
            identifier (typing.Any): The identifier returned from add.
            exc_type (typing.Optional[type]): The exception type to throw into
                the coroutine on cancel. No exception is thrown if nothing is
                given. Instead the coroutine is no longer processed.

        Returns:
            bool: True if the coroutine is cancelled. False if the identifier
                is invalid or if the coroutine is complete.
        """
        raise NotImplementedError()