<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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()