adds a span-based annotation layer as a <tier> to the Exmaralda <body>.

        Parameter
        ---------
        docgraph : DiscourseDocumentGraph
            the document graph from which the chains will be extracted
        body : etree._Element
            an etree representation of the <basic_body> element (and all its
            descendants) of the Exmaralda file
        annotation_layer : str
            the name of a layer, e.g. 'tiger', 'tiger:token' or 'mmax:sentence'

def __add_annotation_tier(self, docgraph, body, annotation_layer):
        """
        adds a span-based annotation layer as a <tier> to the Exmaralda <body>.

        Parameter
        ---------
        docgraph : DiscourseDocumentGraph
            the document graph from which the chains will be extracted
        body : etree._Element
            an etree representation of the <basic_body> element (and all its
            descendants) of the Exmaralda file
        annotation_layer : str
            the name of a layer, e.g. 'tiger', 'tiger:token' or 'mmax:sentence'
        """
        layer_cat = annotation_layer.split(':')[-1]
        temp_tier = self.E('tier',
                           {'id': "TIE{}".format(self.tier_count),
                            'category': layer_cat, 'type': "t",
                            'display-name': "[{}]".format(annotation_layer)})
        self.tier_count += 1

        for node_id in select_nodes_by_layer(docgraph, annotation_layer):
            span_node_ids = get_span(docgraph, node_id)
            if span_node_ids:
                start_id, end_id = self.__span2event(span_node_ids)
                event_label = docgraph.node[node_id].get('label', '')
                event = self.E('event',
                               {'start': "T{}".format(start_id),
                                'end': "T{}".format(end_id)},
                               event_label)
                temp_tier.append(event)
        body.append(temp_tier)