def flatten_list(list_of_list):
    """Input a list of list, return a list that all items are in a list.

    Parameters
    ----------
    list_of_list : a list of list

    Examples
    --------
    >>> tl.utils.flatten_list([[1, 2, 3],[4, 5],[6]])
    [1, 2, 3, 4, 5, 6]

    """
    return sum(list_of_list, [])
