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

        :return: a set of heading types or an empty array if not restricted
        :rtype: ``osid.type.Type``
        :raise: ``IllegalState`` -- syntax is not a ``HEADING``

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