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