Sets/gets the 'root' flag. Target is root is it directly correspods to some
            variant of a main target.

def root (self, set = None):
        """ Sets/gets the 'root' flag. Target is root is it directly correspods to some
            variant of a main target.
        """
        assert isinstance(set, (int, bool, type(None)))
        if set:
            self.root_ = True
        return self.root_