Basic AST

Okay, it wasn't clear to me whether there are any other classes that use this flag. I'm not sure though whether we'll need this flag -- at the relevant places we'll probably have to distinguish between Integer/Boolean/Void/other classes and not between Value/not value. But you can keep it in if you already have a usage in mind :)

Oh BTW, why do you left shift by 4 and not 2? Reserved bits? :)