Format an iterable as a sequence of comma-separated strings.
    
    To match what ConfigObj expects, a single item list has a trailing comma.

def _iterable_as_config_list(s):
    """Format an iterable as a sequence of comma-separated strings.
    
    To match what ConfigObj expects, a single item list has a trailing comma.
    """
    items = sorted(s)
    if len(items) == 1:
        return "%s," % (items[0],)
    else:
        return ", ".join(items)