Canonical dvdC_opprPred x := OpprPred (dvdC_zmod x).
Canonical dvdC_addrPred x := AddrPred (dvdC_zmod x).
Canonical dvdC_zmodPred x := ZmodPred (dvdC_zmod x).

(* Rational number subset. *)
Lemma ratCK : cancel QtoC CtoQ.