from functools import reduce
import operator


def flatten_lists(lists):
    """ Convert a list of lists to a list (reduce list-depth by 1)
    Parameters
    ----------
    lists: list
        a list of lists

    Returns
    -------
    lists: list
        a list of the "internal items"
    """
    if len(lists) == 0:
        return []
    if type(lists[0]) is list:
        return reduce(operator.add, lists)
    return lists