code: public RString restartGroup(String what) {
      List<Placeholder> all = _placeholders.get(what);
      assert all.size() == 1;
  
      Placeholder result = all.get(0);
      if( result.group == null ) {
        throw new NoSuchElementException("Element " + what + " is not a group.");
      }
      result.group.clear();
      return result.group;
    }
docs: can be filled again.