code: def get_document(self, doc_id):
          
          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
docs: given a document ID, returns a merged document graph containng all
          available annotation layers.