given a document ID, returns a merged document graph containng all
        available annotation layers.

def get_document(self, doc_id):
        """
        given a document ID, returns a merged document graph containng all
        available annotation layers.
        """
        layer_graphs = []
        for layer_name in self.layers:
            layer_files, read_function = self.layers[layer_name]
            for layer_file in layer_files:
                if fnmatch.fnmatch(layer_file, '*{}.*'.format(doc_id)):
                    layer_graphs.append(read_function(layer_file))

        if not layer_graphs:
            raise TypeError("There are no files with that document ID.")
        else:
            doc_graph = layer_graphs[0]
            for layer_graph in layer_graphs[1:]:
                doc_graph.merge_graphs(layer_graph)
        return doc_graph