code: def _get_xref(self, line):
          
          # Ex: Wikipedia:Zygotene
          # Ex: Reactome:REACT_22295 "Addition of a third mannose to ..."
          mtch = self.attr2cmp['xref'].match(line)
          return mtch.group(1).replace(' ', '')
docs: Given line, return optional attribute xref value in a dict of sets.