code: def match_set(self, tokens, item):
          
          match, = tokens
          self.add_check("_coconut.isinstance(" + item + ", _coconut.abc.Set)")
          self.add_check("_coconut.len(" + item + ") == " + str(len(match)))
          for const in match:
              self.add_check(const + " in " + item)
docs: Matches a set.