datatype 'label btree =
         Empty |
         Node of 'label * 'label btree * 'label btree;

     fun lookup lt Empty x = false
     |   lookup lt (Node(y,left,right)) x =
             if lt(x,y) then lookup lt left x
             else if lt(y,x) then lookup lt right x
             else (* x=y *) true;

     val lookupS = lookup (op <) (
			Node(16,
			    Node(4,
				Node(1,Empty,Empty),
				Node(9,Empty,Empty)
			    ),
			    Node(36,
				Node(25,Empty,Empty),
				Empty
			    )
			)
		    );