proposition: Lemma f_eqn x: f2 x = f1 (fun y _ => f2 y).
proof: Proof. move: x; apply: (Fix_eq lt_wf). case => //; case => //n p p' Hp. rewrite /f1 Hp; case: (p' n.+1 (ltnSn n.+1)) => y Hy /=. by rewrite Hp. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)