∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t₁ t₂ : Std.ExtDTreeMap α (fun x => β) cmp}
  [inst : Std.TransCmp cmp] {k : α} {h_mem : k ∈ t₁ \ t₂},
  Std.ExtDTreeMap.Const.get (t₁ \ t₂) k h_mem = Std.ExtDTreeMap.Const.get t₁ k ⋯