∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α} {m₁ m₂ : Std.DHashMap α β} [EquivBEq α]
  [LawfulHashable α] {k fallback : α}, k ∉ m₂ → (m₁ \ m₂).getKeyD k fallback = m₁.getKeyD k fallback