code: public ElemTemplateElement appendChild(ElemTemplateElement elem)
    {
      // cannot have content and select
      if (m_selectPattern != null)
      {
        error(XSLTErrorResources.ER_CANT_HAVE_CONTENT_AND_SELECT, 
            new Object[]{"xsl:" + this.getNodeName()});
        return null;
      }
      return super.appendChild(elem);
    }
docs: Add a child to the child list. If the select attribute
  is present, an error will be raised.
  
  @param elem New element to append to this element's children list
  
  @return null if the select attribute was present, otherwise the
  child just added to the child list