code: def _get_synonym(self, line):
          
          mtch = self.attr2cmp['synonym'].match(line)
          text, scope, typename, dbxrefs, _ = mtch.groups()
          typename = typename.strip()
          dbxrefs = set(dbxrefs.split(', ')) if dbxrefs else set()
          return self.attr2cmp['synonym nt']._make([text, scope, typename, dbxrefs])
docs: Given line, return optional attribute synonym value in a namedtuple.
  
          Example synonym and its storage in a namedtuple:
          synonym: "The other white meat" EXACT MARKETING_SLOGAN [MEAT:00324, BACONBASE:03021]
            text:     "The other white meat"
            scope:    EXACT
            typename: MARKETING_SLOGAN
            dbxrefs:  set(["MEAT:00324", "BACONBASE:03021"])
  
          Example synonyms:
            "peptidase inhibitor complex" EXACT [GOC:bf, GOC:pr]
            "regulation of postsynaptic cytosolic calcium levels" EXACT syngo_official_label []
            "tocopherol 13-hydroxylase activity" EXACT systematic_synonym []