I have a habit, when a concept isn't working for me, to repeatedly
   access it in different ways, so that eventually my brain might get the
   picture and respond reliably.  That isn't always working that well for
   me any more, but it is just something that I am so familiar with.
   When engaging this, I get confused when i think of the stats concept of
   "combinations".  i'm not sure they're relevant here, but I can barely
   understand what they are.  I think I