proposition: Lemma zle_diffP1 a b: intp a -> intp b -> (\0z <=z (b -z a) <-> a <=z b).
proof: Proof. move => pa pb; move:(zle_diffP pa pb)(@zle0xP (b -z a)) => sa sb. by split; [ move /sb/sa | move /sa/sb ]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)