intro k
case h
Q : ℕ → Prop
inst✝ : DecidablePred Q
n : ℕ
hn : ∀ (k : ℕ), Q k → k ≤ n
⊢ ∀ (k : ℕ), Q k ↔ k ∈ filter Q (range (n + 1))
case h
Q : ℕ → Prop
inst✝ : DecidablePred Q
n : ℕ
hn : ∀ (k : ℕ), Q k → k ≤ n
k : ℕ
⊢ Q k ↔ k ∈ filter Q (range (n + 1))