English
The i-th element of l.attach equals the i-th element of l, by underlying indexing.
Русский
i-й элемент l.attach равен i-му элементу l, по базовым индексам.
LaTeX
$$$ \\forall i < |l|_\\text{attach},\\ (l.\\text{attach}.get i).1 = l.\\text{get} \\langle i, \\text{length\\_attach}(l) \\rangle $$$
Lean4
theorem get_attach (l : List α) (i) : (l.attach.get i).1 = l.get ⟨i, length_attach (l := l) ▸ i.2⟩ := by simp