code: private void appendGroupingAttribute(final XMLUtil util, final Appendable appendable) throws IOException {
          if (this.grouping) util.appendAttribute(appendable, "number:grouping", "true");
      }
docs: Append the 19.348 number:grouping attribute. Default = false.
  
  @param util       a util for XML writing
  @param appendable where to write
  @throws IOException If an I/O error occurs