def flatten(a_list_of_lists):
    """
    Flattens a list of lists into a single list.

    :param a_list_of_lists:     The list of list to flatten.
    :return:                    The flattened list.
    """

    flattened_list = []

    for sub_list in a_list_of_lists:
        for element in sub_list:
            flattened_list.append(element)

    return flattened_list