type expr =
  | Constant of int
  | Variable of string
  | Add of expr * expr
  | Zero

let rec simplify expr =
  let (c, expr) = simplify_acc expr in
  if c = 0 then expr else Add(Constant c, expr)
and simplify_acc = function
  | Constant x -> (x, Zero)
  | Variable v -> (0, Variable v)
  | Add (l, r) -> simplify_add(simplify_acc l, simplify_acc r)
  | Zero -> (0, Zero)
and simplify_add = function
  | ((_, Constant _), _) |(_, (_, Constant _)) -> failwith "no"
  | ((lc, Zero), (rc, v))
  | ((lc, v), (rc, Zero)) -> (lc + rc, v)
  | ((lc, l), (rc, r)) -> (lc + rc, Add(l, r))