∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.HashMap α β} [EquivBEq α] [LawfulHashable α]
  {f : α → Bool} {k : α}, (Std.HashMap.filter (fun k x => f k) m).getKey? k = Option.filter f (m.getKey? k)