∀ {α : Type u} {β : Type v} [inst : BEq α] [inst_1 : Hashable α] {m : Std.HashMap.Raw α β} [EquivBEq α]
  [LawfulHashable α] [inst_4 : Inhabited α] {f : α → Bool} {k : α},
  m.WF → (Std.HashMap.Raw.filter (fun k x => f k) m).getKey! k = (Option.filter f (m.getKey? k)).get!