let rec eval (vtab : SymTab) (e : EXP) : VALUE =
  let apply op n m =
    match (eval vtab n, eval vtab m) with
      | (INT a, INT b) -> INT (op a b)
  match e with
   | CONSTANT v -> v
   | VARIABLE v -> lookup v vtab
   | ADD (n, m) -> apply (+) n m
   | MULTIPLY (n, m) -> apply (*) n m
   | SUBTRACT (n, m) -> apply (-) n m
   | DIVIDE (n, m) -> apply (/) n m
   | LET_IN (var, e', e) ->
    let vtab = bind var (eval vtab e') vtab
    eval vtab e