i think the smushable rule would work for starch, too. sometimes it seems like heather's diet treats SF and starch as the same thing. SF is proscribed for two uses, too - to create the happy 'gel' in your colon to help D and C (mostly SFS's do this), and to get your digestion going easily before you have harder to digest foods that could incite an attack (this is where low-SF foods that are mostly starch, like white bread, work, too).
this is mostly personal deduction, but also a little research... it seems to make sense, though i wish heather would address the lack of SF in the safe 'cushion' foods like white rice and bread that she refers to as SF foods.