∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {m : Std.ExtDHashMap α fun x => β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {f : α → β → Bool} {k k' : α} {fallback : β},
  m.getKey? k = some k' →
    Std.ExtDHashMap.Const.getD (Std.ExtDHashMap.filter f m) k fallback =
      (Option.filter (fun x => f k' x) (Std.ExtDHashMap.Const.get? m k)).getD fallback