∀ {α : Type u} {β : α → Type v} {m : Std.DHashMap.Raw α β} [inst : BEq α] [inst_1 : Hashable α] [EquivBEq α]
  [LawfulHashable α], m.WF → ∀ {a fallback : α}, m.contains a = false → m.getKeyD a fallback = fallback