type tree = Node of string * exp * tree * tree | Empty

and exp = Etree of tree |  EInt of int  | List of (string*exp);;

let rec evalNode.......



and rec eval (e:exp) =
(match e with
     | EInt _ -> e
     | Etree(tree) -> (match tree with
         | Empty -> []
         | Node(s,i,t1,t2) -> (evalNode (tree)) )

  )