intros hforward x hx
case mp
α : Type u_1
p : α → Prop
inst✝ : DecidablePred p
xs : List α
⊢ foldr (fun a r => decide (p a) && r) true xs = true → ∀ (x : α), x ∈ xs → p x
case mp
α : Type u_1
p : α → Prop
inst✝ : DecidablePred p
xs : List α
hforward : foldr (fun a r => decide (p a) && r) true xs = true
x : α
hx : x ∈ xs
⊢ p x