if (root == null) return null;
	public static FindOccurrencesEngine create(IJavaElement root) {		ICompilationUnit unit= (ICompilationUnit)root.getAncestor(IJavaElement.COMPILATION_UNIT);		if (unit != null)			return new FindOccurencesCUEngine(unit);		IClassFile cf= (IClassFile)root.getAncestor(IJavaElement.CLASS_FILE);		if (cf != null)			return new FindOccurencesClassFileEngine(cf);		return null;	}