exact absurd (hb rfl) Bool.noConfusion
case f.false.zero
ih : size zero ≤ zero
hb : zero = 0 → false = true
⊢ 1 ≤ zero

case f.false.succ
n✝ : ℕ
ih : size (succ n✝) ≤ succ n✝
hb : succ n✝ = 0 → false = true
⊢ 1 ≤ succ n✝
case f.false.succ
n✝ : ℕ
ih : size (succ n✝) ≤ succ n✝
hb : succ n✝ = 0 → false = true
⊢ 1 ≤ succ n✝