main method to create an RSTTree from the output of get_rs3_data().

        TODO: add proper documentation

def dt(self, start_node=None):
        """main method to create an RSTTree from the output of get_rs3_data().

        TODO: add proper documentation
        """
        if start_node is None:
            return self.root2tree(start_node=start_node)

        elem_id = start_node
        if elem_id not in self.elem_dict:
            return []

        elem = self.elem_dict[elem_id]
        elem_type = elem['element_type']

        assert elem_type in ('segment', 'group')

        if elem_type == 'segment':
            return self.segment2tree(
                elem_id, elem, elem_type, start_node=start_node)

        else:
            return self.group2tree(
                elem_id, elem, elem_type, start_node=start_node)