eval1 :: Env -> Expr -> Value
eval1 env (Const b) =
  case b of
    One  -> 1
    Zero -> 0
eval1 :: Env -> Expr -> Value
eval1 env (Const b) =
  case b of
    One  -> 1
    Zero -> 0
eval1 env (Const b) =
  case b of
    One  -> 1
    Zero -> 0