code: public void assertion(boolean b, java.lang.String msg)
    {
  
      if (!b)
      {
        java.lang.String fMsg = XSLMessages.createXPATHMessage(
          XPATHErrorResources.ER_INCORRECT_PROGRAMMER_ASSERTION,
          new Object[]{ msg });
  
        throw new RuntimeException(fMsg);
      }
    }
docs: Tell the user of an assertion error, and probably throw an
  exception.
  
  @param b  If false, a runtime exception will be thrown.
  @param msg The assertion message, which should be informative.
  
  @throws RuntimeException if the b argument is false.