/**
* 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.
*
* @return A string containing the system identifier, or null if none is available.
* @see #getPublicId
*/
public String getSystemId() {
Stylesheet sheet = getStylesheet();
return (sheet == null) ? null : sheet.getHref();
}
