∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m : Std.ExtDHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {a fallback : α}, m.contains a = false → m.getKeyD a fallback = fallback