code: 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.opf 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)
docs: unary expressions