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