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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 _get_xref(self, line):
 """Given line, return optional attribute xref value in a dict of sets."""

        # Ex: Wikipedia:Zygotene
        # Ex: Reactome:REACT_22295 "Addition of a third mannose to ..."
        mtch = self.attr2cmp['xref'].match(line)
        return mtch.group(1).replace(' ', '')