code: private void assertion(boolean b, String msg) throws javax.xml.transform.TransformerException
    {
      if (!b) 
      {
        ErrorListener errorHandler = getErrorListener();
  
        if (errorHandler != null)
        {
          errorHandler.fatalError(
            new TransformerException(
              XSLMessages.createMessage(
                XPATHErrorResources.ER_INCORRECT_PROGRAMMER_ASSERTION,
                new Object[]{ msg }), (SAXSourceLocator)this.getSAXLocator()));
        }
      }
    }
docs: Tell the user of an assertion error, and probably throw an
  exception.
  
  @param b  If false, a TransformerException will be thrown.
  @param msg The assertion message, which should be informative.
  
  @throws javax.xml.transform.TransformerException if b is false.