code: public void executeFallback(
            TransformerImpl transformer)
              throws TransformerException
    {
  
      int parentElemType = m_parentNode.getXSLToken();
      if (Constants.ELEMNAME_EXTENSIONCALL == parentElemType 
          || Constants.ELEMNAME_UNDEFINED == parentElemType)
      {
  
        transformer.executeChildTemplates(this, true);
  
      }
      else
      {
  
        // Should never happen
        System.out.println(
          "Error!  parent of xsl:fallback must be an extension or unknown element!");
      }
    }
docs: Execute the fallback elements.  This must be explicitly called to
  instantiate the content of an xsl:fallback element.
  When an XSLT transformer performs fallback for an instruction
  element, if the instruction element has one or more xsl:fallback
  children, then the content of each of the xsl:fallback children
  must be instantiated in sequence; otherwise, an error must
  be signaled. The content of an xsl:fallback element is a template.
  
  @param transformer non-null reference to the the current transform-time state.
  
  @throws TransformerException