def gather_lists(list_):
    """
    Concatenate all the sublists of L and return the result.

    @param list[list[object]] list_: list of lists to concatenate
    @rtype: list[object]

    >>> gather_lists([[1, 2], [3, 4, 5]])
    [1, 2, 3, 4, 5]
    >>> gather_lists([[6, 7], [8], [9, 10, 11]])
    [6, 7, 8, 9, 10, 11]
    """
    new_list = []
    for l in list_:
        new_list += l
    return new_list
    # equivalent to...
    # return sum([l for l in list_], [])