It is only recently that I have been able to grok what some and many are even about (I think), and they seem to only make sense in cases where executing the Alternative action results in a portion of some input being consumed or not consumed.  "some v" means "consume at least one v and return the list of items consumed or fail", and "many v" means "consume zero or more v and return the list of items consumed or the empty list of none are consume".  It thus makes sense for there to be some subclass of Alternative called something like "Consumptive" that contains these methods.