code: protected List<Selector> groups() throws ParserException {
  
          List<Selector> groups = new ArrayList<Selector>();
          int groupNumber = 0;
          while (!end()) {
              ignoreWhitespaces();
              parserListener.beginGroup(groupNumber, pos);
              Selector selector = selector();
              groups.add(selector);
              parserListener.endGroup(selector);
              groupNumber++;
              ignoreWhitespaces();
              if (!end() && current != ',') {
                  throw new ParserException("There is something left in the selector at position " + pos);
              }
              next();
          }
          return groups;
  
      }
docs: selectors_group
  : selector [ COMMA S* selector ]*