<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate_unset_command(self, line: str, position: int, annotation: str) -> None: """Raise an exception when trying to ``UNSET X`` if ``X`` is not already set. :raises: MissingAnnotationKeyWarning """

        if annotation not in self.annotations:
            raise MissingAnnotationKeyWarning(self.get_line_number(), line, position, annotation)