code: def rebase_opt(self):
          
          if not hasattr(self, '_rebase_opt'):
              # out = b"MAJOR.MINOR.REVISION" // b"3.4.19" or b"4.0.0"
              out, err = Popen(
                  ['cleancss', '--version'], stdout=PIPE).communicate()
              ver = int(out[:out.index(b'.')])
              self._rebase_opt = ['--root', self.root] if ver == 3 else []
          return self._rebase_opt
docs: Determine which option name to use.