It is something of a design feature of pydoctor's data model that you
can crawl from anywhere to anywhere.  This is mostly very handy, but
it means that there are veeeeeeery long chains of references around,
and if pickle or cPickle happen to choose one of these when pickling
the system you can easily get recursion limit exceeded errors.  So, we
need to give pickle a hand, which is what Documentable.__getstate__
and System.__setstate__ are all about.

Documentable.__getstate__ essentially replaces references to other
Documentable objects with references to their fullNames().  These
can't easily be resolved in a hypothetical Documentable.__setstate__
because the referenced Documentables probably don't exist yet (all
Documentables are in cycles involving other Documentables except in
edge cases; consider 'd.parent.contents[d.name] is d' for starters).
So I sort all this out in System.__setstate__ (this probably means
that if you try to pickle a Documentable directly it won't unpickle
correctly; don't care very much).

Trying to debug/fix this whilst on/recovering from a long haul flight
was probably not the best idea.