proposition: Lemma Z_of_BZ_pa x: intp x -> (BZ_of_Z (Z_of_BZ x)) = x.
proof: Proof. move => H; exact:(chooseT_pr nonempty_int (BZ_of_Z_surjective H)). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype order) mathcomp(ssrnat ssrint ssralg ssrnum div)