proposition: Lemma f_eqn x: f2 x = f1 (n:=x) (fun y _ => f2 y).
proof: Proof. move: x; apply: (Fix_eq lt_wf) => n A B Hp; apply: psum_exten. move => k _; rewrite /extension;case: (lt_dec k n) => // a; apply: Hp. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)