is this written down anywhere? my guess would be that it is an adaptation of this comment Plasma Cash: Plasma with much less per-user data checking - #18 by MaxC to MVP, but I’d rather prefer to encourage more stuff to be written down.
I agree with the need for this limbo exit mechanism, but there’s some stuff in the “problem and motivation” section that concerns me, in particular, the use of bonds. IMO it should never be OK for an honest user to end up in a situation where his funds are on a plasma chain and exiting could result in him losing a large bond. a trivial way to enforce this property is to not require bonds for any exits, but this means that anyone can grief anyone else (unbounded amount, but bounded griefing factor). if we accept this, then the user-might-have-to-exit-twice problem is actually acceptable. of course, we probably don’t want to accept this. I think the general principle should be that users post bonds to exit but the bond is only taken away in cases where the exiter is provably cheating.