English
For any xs, xs.sym 1 equals xs.map (cons ? to the nil tail).
Русский
Для любого xs xs.sym 1 равно xs.map (cons ? к nil).
LaTeX
$$$$ xs.sym\\,1 = xs.map (\\lambda x. x ::\\! Nil) $$$$
Lean4
theorem sym_one_eq : xs.sym 1 = xs.map (· ::ₛ .nil) := by
induction xs with
| nil => simp only [List.sym, Nat.succ_eq_add_one, Nat.reduceAdd, map_nil]
| cons x xs ih => rw [map_cons, ← ih, List.sym, List.sym, map_singleton, singleton_append]