@abc.abstractmethod<EOL><INDENT>def get_calendar_types(self):<DEDENT>
return<EOL>
Gets the set of acceptable calendar types.

        :return: the set of calendar types
        :rtype: ``osid.type.Type``
        :raise: ``IllegalState`` -- syntax is not a ``DATETIME`` or ``DURATION``

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