def parse(code):
    """Annotate user code.
    Return annotated code (str) if annotation detected; return None if not.
    code: original user code (str)
    """
    try:
        ast_tree = ast.parse(code)
    except Exception:
        raise RuntimeError('Bad Python code')

    transformer = Transformer()
    try:
        transformer.visit(ast_tree)
    except AssertionError as exc:
        raise RuntimeError('%d: %s' % (ast_tree.last_line, exc.args[0]))

    if not transformer.annotated:
        return None

    last_future_import = -1
    import_nni = ast.Import(names=[ast.alias(name='nni', asname=None)])
    nodes = ast_tree.body
    for i, _ in enumerate(nodes):
        if type(nodes[i]) is ast.ImportFrom and nodes[i].module == '__future__':
            last_future_import = i
    nodes.insert(last_future_import + 1, import_nni)

    return astor.to_source(ast_tree)