from itertools import chain


def flatten(listOfLists):
    """
    Flatten one level of nesting given a list of lists. That is, convert
    [[1, 2, 3], [4, 5, 6]] to [1, 2, 3, 4, 5, 6].

    :param listOfLists: a list of lists, obviously
    :return: the flattened list
    """

    return list(chain.from_iterable(listOfLists))