code: public String getSystemId()
    {
      Stylesheet sheet=getStylesheet();
      return (sheet==null) ? null : sheet.getHref();
    }
docs: Return the system identifier for the current document event.
  
  <p>If the system identifier is a URL, the parser must resolve it
  fully before passing it to the application.</p>
  
  @return A string containing the system identifier, or null
  if none is available.
  @see #getPublicId