∀ {α : Type u} [inst : BEq α] [inst_1 : Hashable α] {m₁ m₂ : Std.HashSet.Raw α} [inst_2 : Inhabited α] [EquivBEq α]
  [LawfulHashable α], m₁.WF → m₂.WF → ∀ {k : α}, k ∉ m₂ → (m₁ ∪ m₂).get! k = m₁.get! k