One of the most common, and legitimate, criticisms of the music of Stephen Sondheim by performers and critics alike is that he's not tuneful enough. "Hummable," I think is the word most often used by these critics. And yet somehow, a week after seeing The Arden Theatre's production of Assassins, I simply can't get the music out of my head. It may not be hummable, but goddamn if it isn't memorable. That's a sure......