∀ {α : Type u} {β : α → Type v} [inst : BEq α] (a : α) (x : List ((a : α) × β a)),
  Std.Internal.List.getKey? a x =
    match x with
    | [] => none
    | ⟨k, snd⟩ :: l => bif k == a then some k else Std.Internal.List.getKey? a l