Each game fork has its own rules. Additional forks may be possible if the particular game would allow it at the time. Reunifications must be legal in all affected forks.
I wouldn't hold your breath, necessarily. As mentioned I did once get about 20% of the way through a move, but if I don't do it all in a very short period it usually means I have to start from scratch again.
Heh, almost three months later I answer your question Tuj: "Where's my yoghurt?" has a very similar relationship to the concept of "rules" as does the game of Mornington Crescent. Or to put it another way I made it up. Still, it got us talking for 6 moves. Maybe I should invent the game then... erm...
Okay everyone, I've hidden my yoghurt. I'll do a short and probably obscene mime of where I put it, then you can start guessing... <mime> Waves hands around slowly and stands on one leg. Falls over. </mime> Okay, start guessing. Yoghurt search on.