obtain ⟨i, hi⟩ := hx
α : Type u_1
inst✝ : DecidableEq α
as : ArraySet α
x : α
hx : ∃ n, List.get as.content.data n = x
⊢ ∃ i, as.content[i] = x
case intro
α : Type u_1
inst✝ : DecidableEq α
as : ArraySet α
x : α
i : Fin (List.length as.content.data)
hi : List.get as.content.data i = x
⊢ ∃ i, as.content[i] = x