def secret_sort(secret_index):
    """
    Return a lambda that takes one argument, a list of lists of an unknown amount of numbers. The lambda should return the list of lists, but sorted by sublist[secret_index].
    
    :param secret_index: secret index to sort by (integer)
    :return: lambda that takes a list of lists of numbers and sorts them
    """
    return lambda l: sorted(l, key=lambda x: x[secret_index])