takes a document graph, converts it into GraphML format and writes it to
    a file.

def write_graphml(docgraph, output_file):
    """
    takes a document graph, converts it into GraphML format and writes it to
    a file.
    """
    dg_copy = deepcopy(docgraph)
    layerset2str(dg_copy)
    attriblist2str(dg_copy)
    remove_root_metadata(dg_copy)
    nx_write_graphml(dg_copy, output_file)