data Expr = Lit Int | Add Expr Expr | Mul Expr Expr | Neg Expr

match simplify (e : Expr) : Expr =
  | Add (Lit 0) x -> x
  | Add x (Lit 0) -> x
  | Mul (Lit 0) x -> Lit 0
  | Mul x (Lit 0) -> Lit 0
  | Mul (Lit 0) (Lit 1) -> Lit 0
  | Mul (Lit 1) x -> x
  | Mul x (Lit 1) -> x
  | Neg (Neg x) -> x
  | e -> e