So, with the bin filling decisions, do we want hoe many ordered lists
   there are, or how many unordered sets there are?
   Changing the order of the decisions doesn't make a new possible
   outcome.  To calculate how many possible outcomes there are, we keep
   the decision order fixed.
   So, we want combinations here.
   The number of ways to fill a histogram bin to binheight appears to be
   (sampcount C binheight) popcount ^ (sampcount - binheight), but I'm not
   sure !