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