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.
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.