have hmem : x ∈ [x] := by constructor
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
⊢ (match !decide (x ∈ [x]) with
    | true => x :: filter (fun x_1 => !decide (x_1 ∈ [x])) xs
    | false => filter (fun x_1 => !decide (x_1 ∈ [x])) xs) =
    filter (fun x_1 => !decide (x_1 ∈ [x])) xs
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
hmem : x ∈ [x]
⊢ (match !decide (x ∈ [x]) with
    | true => x :: filter (fun x_1 => !decide (x_1 ∈ [x])) xs
    | false => filter (fun x_1 => !decide (x_1 ∈ [x])) xs) =
    filter (fun x_1 => !decide (x_1 ∈ [x])) xs