  Simple MCBR models of chemical evolution are extended to the limit of
dominant gas inflow or outflow with respect to gas locked up into long-lived
stars and remnants. For an assigned empirical differential oxygen abundance
distribution, which can be linearly fitted, a family of theoretical curves is
built up with assigned prescriptions. For curves with increasing cut parameter,
the gas mass fraction locked up into long-lived stars and remnants is found to
attain a maximum and then decrease towards zero as the flow tends to infinity,
while the remaining parameters show a monotonic trend. The theoretical integral
oxygen abundance distribution is also expressed. An application is performed to
the empirical distribution deduced from two different samples of disk stars,
for both the thin and the thick disk. The constraints on formation and
evolution are discussed in the light of the model. The evolution is tentatively
subdivided into four stages, A, F, C, E. The empirical distribution related to
any stage is fitted by all curves for a wide range of the cut parameter. The F
stage may safely be described by a steady inflow regime, implying a flat
theoretical distribution, in agreement with the results of hydrodynamical
simulations. Finally, (1) the change of fractional mass due to the extension of
the linear fit to the empirical distribution, towards both the (undetected)
low-metallicity and high-metallicity tail, is evaluated and (2) the idea of a
thick disk-thin disk collapse is discussed, in the light of the model.
