def METHOD_NAME(arg_list, arg_map):
    # Merge the relative_labels= and relative_labels_map= arguments as seen in
    # the public macros below.  The result is a map where the arg_list items
    # are twinned into matching key:key pairs, union'd with the arg_map.  (Also
    # double-checks that the keys and values are all relative labels.)
    result = dict([(x, x) for x in arg_list])
    result.update(arg_map)
    for x in result.keys() + result.values():
        if not x.startswith(":"):
            fail("Expected relative_label, got " + x)
    return result