Expander: non deterministic expansion on some circular ref patterns

[fredbi]

Let's make things official about this limitation, mentioned in the discussion following #75 (credits @elakito):

• when $ref do form several cycles (i.e. out of the root document)
• when some of those cycles have an intersection

then the expander chose some random expansion path from the common cycle

This behavior is illustrated by this fixture (not run by tests).

[magodo]

@fredbi Is there any progress on this issue?

[fredbi]

@magodo no. I've worked a bit on it last year then gave up since I found it very difficult to solve.