∀ {α : Type u} {β : α → Type v} [inst : BEq α] [inst_1 : Hashable α] {m₁ m₂ : Std.DHashMap.Raw α β} [EquivBEq α]
  [LawfulHashable α], m₁.WF → m₂.WF → ∀ {k fallback : α}, k ∉ m₂ → (m₁ \ m₂).getKeyD k fallback = m₁.getKeyD k fallback