code: @Override
      public boolean isValid(TagData us) {
          if (!Util.isSpecified(us, ITEMS)) {
              if (!Util.isSpecified(us, BEGIN) || !(Util.isSpecified(us, END))) {
                  return false;
              }
          }
          return true;
      }
docs: /*
  Currently implements the following rules:
  
  - If 'items' is not specified, 'begin' and 'end' must be