Stable Fin (n + 1) head index #
Compared Scott permutation/cancellation conditions mention the distinguished
index 0 : Fin (n + 1). Elaborating that literal attaches a generated
._proof_N to whichever declaration first needs OfNat. Palomar compares
those generated constants, so Challenge and Solution must share a named
proof of 0 < n + 1 instead.
The distinguished index in a nonempty Scott sequence of length n + 1.