proposition: Lemma nth_error_set_nth_rcons n a b c : nth_error st n = Some a -> set_nth def (rcons st c) n b = rcons (set_nth def st n b) c.
proof: Proof. by elim: n st => [|n IH] [|d st'] //= /IH ->. Qed.
imports: mathcomp(all_ssreflect)