def METHOD_NAME(self, subtask: Subtask):
    """
    Check whether a subtask is a mapper subtask. Note the even this a mapper subtask, it can be a reducer subtask
    at the same time such as `DuplicateOperand`, see
    https://user-images.githubusercontent.com/12445254/174305282-f7c682a9-0346-47fe-a34c-1e384e6a1775.svg
    """
    return subtask in self._mapper_indices