∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {γ : Type w} {m : Std.DHashMap α fun x => β} [EquivBEq α]
  [LawfulHashable α] {f : α → β → Option γ},
  (Std.DHashMap.filterMap f m).isEmpty = true ↔
    ∀ (k : α) (h : k ∈ m), f (m.getKey k h) (Std.DHashMap.Const.get m k h) = none