My question is that if you mutate a patch inside the block argument to
#patchesDo: are you promised that the next patch evaluated will see the
"old" state of the world?

If the patches in a world are supposed to be executed *in parallel*, then
the fact that we are actually evaluating them sequentially shouldn't
affect the results.

Nothing I see seems to make this so. I would think some sort of
"currentState/nextState" setup would be needed, with #get: pulling from
the currentState and #set: setting into the nextState.

Cheers,
Bijan Parsia.