case succ
x : Nat
ih : isEven (2 * x) = true
⊢ (match 2 * Nat.succ x with
    | 0 => true
    | Nat.succ n => isOdd n) =
    true
case succ
x : Nat
ih : isEven (2 * x) = true
⊢ (match 2 * x + 1 with
    | 0 => false
    | Nat.succ n => isEven n) =
    true