English
Converts a Fin2 n into a Fin n by mapping fz ↦ 0 and fs i ↦ i.toFin.succ.
Русский
Преобразует Fin2 n в Fin n: fz ↦ 0, fs i ↦ i.toFin.succ.
LaTeX
$$$\text{toFin} : \mathrm{Fin2}(n) \to \mathrm{Fin}(n),\; \text{toFin}(\mathrm{fz}) = 0,\; \text{toFin}(\mathrm{fs}\ i) = \text{(toFin } i)\text{.succ}$$$
Lean4
/-- Converts a `Fin2` into a `Fin`. -/
def toFin {n : Nat} (i : Fin2 n) : Fin n :=
match i with
| fz => 0
| fs i => i.toFin.succ