[PEAK] Re: trellis.Set.discard

[Sergey Schetinin]

> they shouldn't change *anything* else
> in our existing cells.

Correction, anything in cells already participating (changed or read)
in parent transaction. Involving some other cells doesn't break
anything.

Futures can be accessed from many rules, is this required?
Effectively, reading a future is writing some cell, so why is it
allowed? And if a future is only read by one rule at a time, complete
rollback is not that different from usual value change undo. If
there's genuine need for futures used by multiple rules, then maybe
each rule should get it's own future which would get merged (and
possibly checked for conflicts as well) before putting the data into
the todo cell?