@classmethod
def parse(cls, url):
    """Parses a :term:`VRI` and creates a :class:`VUrl` reference.

        :param url: a :term:`VRI` string for the resource
        :type  url: unicode
        :returns:   :class:`VUrl` identifying the resource.

        This is an abstract method, derived classes must implement.

        """
    raise NotImplementedError()
