∀ {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] [inst : Inhabited β] {k : α},
  k ∉ m₂ → Std.DHashMap.Const.get! (m₁.union m₂) k = Std.DHashMap.Const.get! m₁ k