Towards a proper understanding of congruences and quotients: - noticed thet the inclusion of the natural numbers N into the integers Z is mono and epi in the category of monoids, but ic of course not iso - introduced regular/strong/extremal epis between split epis (=retractions) and plain epis; the strong ones that are also monos are precisely the isos; hence strong epis in categories of structured sets with structure-preserving functions as morphisms will be surjective - compared split/strong/extremal epis (in the section on special morphisms) - compared split/regular/strong epis (in the section on (co)limits, since the definition of regular epis utilizes coequalizers)