code: @Override
    protected Object doExec(Element element, Object scope, String format, Object... arguments) throws IOException
    {
      Stack<Index> indexes = this.serializer.getIndexes();
      if(indexes.size() == 0) {
        throw new TemplateException("Required ordered collection index is missing. Numbering operator cancel execution.");
      }
      this.serializer.writeTextContent(getNumbering(this.serializer.getIndexes(), format));
      return null;
    }
docs: Insert formatted numbering as element text content. If serializer indexes stack is empty throws templates exception;
  anyway, validation tool running on build catches numbering operator without surrounding ordered list.
  
  @param element element declaring numbering operator,
  @param scope scope object,
  @param format numbering format, see class description for syntax,
  @param arguments optional arguments, not used.
  @return always returns null to signal full processing.
  @throws TemplateException if serializer indexes stack is empty.
  @throws IOException if underlying writer fails to write.