@abc.abstractmethod<EOL><INDENT>def is_root(self):<DEDENT>
return<EOL>
Tests if this node is a root in the hierarchy (has no parents).

        A node may have no more parents available in this node structure
        but is not a root in the hierarchy. If both ``is_root()`` and
        ``has_parents()`` is false, the parents of this node may be
        accessed thorugh another node structure retrieval.

        :return: ``true`` if this node is a root in the hierarchy, ``false`` otherwise
        :rtype: ``boolean``


        *compliance: mandatory -- This method must be implemented.*