def __get_segment_types(self, element):
        """
        given a <segment> or <group> element, returns its segment type and the
        segment type of its parent (i.e. its dominating node)

        Parameters
        ----------
        element : ??? etree Element

        Returns
        -------
        segment_type : str
            'nucleus', 'satellite' or 'isolated' (unconnected segment, e.g. a
            news headline) or 'span' (iff the segment type is currently
            unknown -- i.e. ``relname`` is ``span``)
        parent_segment_type : str or None
            'nucleus', 'satellite' or None (e.g. for the root group node)
        """
        if not 'parent' in element.attrib:
            if element.tag == 'segment':
                segment_type = 'isolated'
                parent_segment_type = None
            else:  # element.tag == 'group'
                segment_type = 'span'
                parent_segment_type = None
            return segment_type, parent_segment_type

        # ``relname`` either contains the name of an RST relation or
        # the string ``span`` (iff the segment is dominated by a span
        # node -- a horizontal line spanning one or more segments/groups
        # in an RST diagram). ``relname`` is '', if the segment is
        # unconnected.
        relname = element.attrib.get('relname', '')
        # we look up, if ``relname`` represents a regular, binary RST
        # relation or a multinucular relation. ``reltype`` is '',
        # if ``relname`` is ``span`` (i.e. a span isn't an RST relation).
        reltype = self.relations.get(relname, '')

        if reltype == 'rst':
            segment_type = 'satellite'
            parent_segment_type = 'nucleus'
        elif reltype == 'multinuc':
            segment_type = 'nucleus'
            parent_segment_type = None # we don't know it's type, yet
        else:  # reltype == ''
            # the segment is of unknown type, it is dominated by
            # a span group node
            segment_type = 'span'
            parent_segment_type = 'span'
        return segment_type, parent_segment_type