∀ {α : Type u} {β : α → Type v} [inst : BEq α] [inst_1 : Hashable α] {m₁ m₂ : Std.DHashMap.Internal.Raw₀ α β}
  [EquivBEq α] [LawfulHashable α],
  (↑m₁).WF → (↑m₂).WF → ∀ {k : α}, m₂.contains k = false → (m₁.union m₂).getKey? k = m₁.getKey? k