fact: Fixpoint T2eq x y {struct x} := match x, y with | zero, zero => true | cons a b n c, cons a' b' n' c' => [&& T2eq a a', T2eq b b', n== n' & T2eq c c' ] | _, _ => false end.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)