  When the term ``black hole'' was originally coined in 1968, it was
immediately conjectured that the only pure vacuum equilibrium states were those
of the Kerr family. Efforts to confirm this made rapid progress during the
``classical phase'' from 1968 to 1975, and some gaps in the argument have been
closed during more recent years. However the presently available demonstration
is still subject to undesirably restrictive assumptions such as non-degeneracy
of the horizon, as well as analyticity and causality in the exterior.
