code: @Override
    public void setRefid(final Reference r) throws BuildException {
      if (!this.defineList.isEmpty()) {
        throw tooManyAttributes();
      }
      super.setRefid(r);
    }
docs: Specifies that this element should behave as if the content of the
  element with the matching id attribute was inserted at this location. If
  specified, no other attributes or child content should be specified,
  other than "description".