def singletonize(groups):
    """Singletonize each element of the contained lists.

    Example: [[1,2],[3],[4,5,6]] -> [[1],[2],[3],[4],[5],[6]]
    :param groups: list of lists to be singletonized
    :return: singletonized lists
    """
    singles = []
    for group in groups:
        singles.extend([[el] for el in group])
    return singles