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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def handle_unset_evidence(self, line: str, position: int, tokens: ParseResults) -> ParseResults: """Unset the evidence, or throws an exception if it is not already set. The value for ``tokens[EVIDENCE]`` corresponds to which alternate of SupportingText or Evidence was used in the BEL script. :raises: MissingAnnotationKeyWarning """

        if self.evidence is None:
            raise MissingAnnotationKeyWarning(self.get_line_number(), line, position, tokens[EVIDENCE])
        self.evidence = None
        return tokens