code: void appendXMLHelper(final XMLUtil util, final Appendable appendable, final String numberStyleName,
                           final CharSequence number) throws IOException {
          this.appendOpenTag(util, appendable, numberStyleName, this.dataStyle.getName());
          appendable.append(number);
          this.appendCloseTag(appendable, numberStyleName);
  
          if (this.negativeValueColor != null) {
              this.appendOpenTag(util, appendable, numberStyleName, this.dataStyle.getName() + "-neg");
              this.appendStyleColor(util, appendable);
              appendable.append("<number:text>-</number:text>");
              appendable.append(number);
              this.appendStyleMap(util, appendable);
              this.appendCloseTag(appendable, numberStyleName);
          }
      }
docs: A helper to append XML content
  @param util an util
  @param appendable destination
  @param numberStyleName the name of the number syle
  @param number the number itself
  @throws IOException if an I/O error occurs