proposition: Lemma eqCmodDr e x y z : (y + x == z + x %[mod e])%C = (y == z %[mod e])%C.
proof: Proof. by rewrite /eqCmod addrAC opprD !addrA subrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun ssrnat eqtype seq choice) mathcomp(div fintype path bigop finset prime order ssralg) mathcomp(poly polydiv mxpoly generic_quotient countalg) mathcomp(ssrnum closed_field ssrint archimedean rat intdiv) mathcomp(algebraics_fundamentals)