unary expressions

def eval_unx(self, exp):
    "unary expressions"
    inner=self.eval(exp.val)
    if exp.op.op=='+': return inner
    elif exp.op.op=='-': return -inner
    elif exp.op.op=='not': return threevl.ThreeVL.nein(inner)
    else: raise NotImplementedError('unk_op',exp.op)