proposition: Lemma subf_div2 x y: y != 0 -> x / y - 1 = (x - y) /y.
proof: Proof. by move => h; rewrite -opprB subf_div1 // -mulNr opprB. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype order bigop ssralg) mathcomp(div ssrnum ssrint rat prime path binomial) mathcomp(tuple finset) gaia(fibm)