A constraint is some sort of condition on a datum. This module checks one
condition against one value at a time, and I call the thing that checks
that condition the "constraint". A constraint returns true or false, and
that's it. It should have no side effects, it should not change program
flow, and it should mind its own business. Let the thing that calls the
constraint figure out what to do with it. I want something that says "yes"
or "no" (and I discuss why this needs a fancy module later).
