Free Variables Are Back (partially)

I've added some support for free variables back into somerby.net/mack/logic.  They are now treated as constants denoting actual objects.  At the moment, they are not allowed in statements which contain modal operators, since I have not yet found a satisfactory way to deal with constants in modal statements.  Or at least a way that I'm willing to commit to.  I'm working on it.  I've been reading Possible Worlds, "Actualism" and "SQML" and other such things for instruction and insight.  I knew that there was more than one system of modal logic, but this reading has made me appreciate the need for me to define exactly what system of modal logic somerby.net/mack/logic implements.  So I'm working on that, too.