datatype node = Constant of int 
                | Symbol of string 
                | Negate of node 
                | Add of node*node 
                | Subtract of node*node; 

fun tostring (Constant c) = Int.fmt StringCvt.DEC c
  | tostring (Symbol s) = s
  | tostring (Negate n) = "-" ^ tostring(n)
  | tostring (Add (x, y)) = "(" ^ tostring(x) ^ "+" ^ tostring(y) ^ ")"
  | tostring (Subtract (x, y)) = "(" ^ tostring(x) ^ "-" ^ tostring(y) ^ ")";