proposition: Lemma phi_succ a u v n: a < u -> phi a (cons u v 0 (\F n.+1)) = [a, cons u v 0 (\F n)].
proof: Proof. by move => /= ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)