(* For Exercises 6.2.(6-8) *)
datatype tree = Leaf of int | Internal of (int * tree * tree);

(* For Exercises 6.2.(14-17) *)
datatype operation = Plus | Minus | Mul | Div;
datatype expression =  Internal of operation * expression * expression
      | Leaf of int;

fun printOper(Plus) = "+"
  | printOper(Minus) = "-"
  | printOper(Mul) = "*"
  | printOper(Div) = "/";