∀ {α : Type u} [inst : BEq α] [inst_1 : Hashable α] {β : Type v} {m₁ m₂ : Std.DHashMap.Raw α fun x => β} [EquivBEq α]
  [LawfulHashable α], m₁.WF → m₂.WF → ∀ {k : α}, k ∉ m₁ → Std.DHashMap.Raw.Const.get? (m₁.diff m₂) k = none