∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m₁ m₂ : Std.ExtHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {k : α} [inst_2 : Inhabited β], k ∉ m₂ → (m₁ \ m₂).get! k = m₁.get! k