@abc.abstractmethod<EOL><INDENT>def get_version_set(self):<DEDENT>
return<EOL>
Gets the set of acceptable ``Versions``.

        :return: a set of ``Versions`` or an empty array if not restricted
        :rtype: ``osid.installation.Version``
        :raise: ``IllegalState`` -- syntax is not a ``VERSION``

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