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

Lemma ratCK : cancel QtoC CtoQ.