English
In Fin(n+2), the element with value a is equal to 1 precisely when a = 1.
Русский
В Fin(n+2) элемент со значением a равен 1 тогда и только тогда, когда a = 1.
LaTeX
$$$\forall {n a : \mathbb{N}} {ha : a < n + 2},\ 1 = (\langle a, ha\rangle : Fin (n + 2)) \iff a = 1$$$
Lean4
@[simp]
theorem one_eq_mk {n a : Nat} {ha : a < n + 2} : 1 = (⟨a, ha⟩ : Fin (n + 2)) ↔ a = 1 := by simp [eq_comm]