Claifications: visit procs are supplied by the core, users aren't

Claifications:  visit procs are supplied by the core, users aren't
expected to write their own.  A NULL "object" must not be passed to
the visit callback.  A non-zero return from a visit proc isn't
necessarily an error return (and it doesn't matter to the tp_traverse
code *what* it might signify, their only job is to return it).