proposition: Lemma f_eqn x: f2 x = f1 (fun y _ => f2 y).
proof: Proof. move: x; apply: (Fix_eq lte_wf). case => //; case => //; case => //; case => //. move => n p p' Hp; rewrite /f1; case: (odd_dec n) => // on. rewrite Hp; case: (p' n.+2 (f0c on)) => y Hy /=. by case: (f0a on Hy) => a b; rewrite Hp. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)