rw [mem_def, Array.mem_def, List.mem_iff_get] at hx
α : Type u_1
inst✝ : DecidableEq α
as : ArraySet α
x : α
hx : x ∈ as
⊢ ∃ i, as.content[i] = x
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