∀ {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 fallback : α}, m₁.contains k = false → (m₁.union m₂).getKeyD k fallback = m₂.getKeyD k fallback