proposition: Lemma p_pick_cancel : pcancel nat_of_P p_unpickle.
proof: Proof. move=> x; rewrite /p_unpickle; congr Some. by rewrite pred_o_P_of_succ_nat_o_nat_of_P_eq_id. Qed.
imports: mathcomp(ssreflect ssrbool eqtype ssrnat seq order) mathcomp(choice fintype finfun ssrfun bigop ssralg) Zbool Qcanon ZArith