∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {m₁ m₂ : Std.DHashMap α fun x => β} [EquivBEq α]
  [LawfulHashable α] {k : α} {fallback : β},
  k ∉ m₂ → Std.DHashMap.Const.getD (m₁.union m₂) k fallback = Std.DHashMap.Const.getD m₁ k fallback