∀ {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] {k : α}, m₁.WF → m₂.WF → (m₁ ∩ m₂).contains k = (m₁.contains k && m₂.contains k)