datatype ('label) tree =
         Node of 'label * 'label tree list;

     fun sum(Node(a,nil)) = a
     |   sum(Node(a,t::ts)) =
             sum(t) + sum(Node(a,ts));

     val t = Node(3, [
                 Node(4,nil),
                 Node(5, [Node(7,nil)]),
                 Node(6,nil)
             ]);

sum t;