∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m : Std.ExtDHashMap α β} [inst : LawfulBEq α]
  {f : (a : α) → β a → Bool} {k fallback : α},
  (Std.ExtDHashMap.filter f m).getKeyD k fallback =
    ((m.getKey? k).pfilter fun x_2 h' => f x_2 (m.get x_2 ⋯)).getD fallback