∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap α β cmp} [Std.TransCmp cmp] {i : ℕ}
  {fallback : (a : α) × β a}, t₁.Equiv t₂ → t₁.entryAtIdxD i fallback = t₂.entryAtIdxD i fallback