def _pack_list(lst):
    """
    The core method, convert list of strings to one big string.
    Every line in the string will store a single item from list. First
    line will keep a number of items. So items in the list should be a
    strings and not contain "
".\\ This is useful to store a list of
    users IDs in the local file.
    """
    return str(len(lst)) + u'\n' + u'\n'.join(lst)
