proposition: Lemma ex4_modify_r_r x i: inc x ex4_F -> ex4_fct_r (ex4_modify_r x i) = i.
proof: Proof. move => ha; move:(ex4_length_prop1 ha). rewrite /ex4_modify_r/ex4_fct_r /ex4_last; move => [sa sb]. rewrite Lgd LgV//; first by Ytac0; aw. rewrite {2} sb; apply/(NltP (NS_succ sa)); apply:(cltS sa). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)