A search is conducted for a class of PERs (partial equivalence relations on the natural numbers) such that the resulting full subcategory has the expected properties of any good category of CPOs: it should be a CCC (Cartesian closed category) and every endomorphism should have a canonical fixed point. Moreover the reflection functor (usually called the lifting operation) should yield a good notion of partial map. The following topics are discussed: conventions, partial-map classifiers, ExPERS, ExPERS as domains, reflectivity of strict maps, multicorreflectivity of strict maps, the extensional natural numbers, domain equations, and intrinsic descriptions.<<ETX>>