∀ {α : Type u} [inst : BEq α] [inst_1 : Hashable α] {β : Type v} {m₁ m₂ : Std.HashMap.Raw α β} [EquivBEq α]
  [LawfulHashable α] [inst_4 : Inhabited β], m₁.WF → m₂.WF → ∀ {k : α}, (m₁ ∪ m₂).get! k = m₂.getD k (m₁.get! k)