add attributes to segment nodes, as well as edges to/from other
        segments/groups. add segment to list of EDUs.

        Parameters
        ----------
        segment : ??? etree Element

def __add_segment(self, segment):
        """
        add attributes to segment nodes, as well as edges to/from other
        segments/groups. add segment to list of EDUs.

        Parameters
        ----------
        segment : ??? etree Element
        """
        segment_id = self.ns+':'+segment.attrib['id']
        self.segments.append(segment_id)
        segment_type, parent_segment_type = self.__get_segment_types(segment)
        segment_text = sanitize_string(segment.text)
        segment_label = get_segment_label(
            segment, segment_type, segment_text, self.ns, self.tokenized)

        self.add_node(
            segment_id, layers={self.ns, self.ns+':segment'},
            attr_dict={'label': segment_label,
                       self.ns+':text' : segment_text,
                       self.ns+':segment_type': segment_type})

        # store RST segment in list of EDUs
        self.edus.append(segment_id)

        if 'parent' in segment.attrib:
            self.__add_parent_relation(segment, segment_id, segment_type,
                                       parent_segment_type)