  Considerable progress has been made in understanding the hydrodynamics, but
only to a certain extent the magnetohydrodynamics, of shaping bipolar outflows
forming protoplanetary nebulae (PPNs) and planetary nebulae (PNs). In
particular, Blackman et al. (2001b,2001a) point out two problems related to the
formation of PNs and PPNs, regarding the formation of multipolar structures and
the origin of the nebulae. They propose a solution by giving a
semi-quantitative physical model which should be investigated by numerical
simulations.
