[(1, 2, 3), (2, 3, 4), (3, 4, 5)]

    When the window is larger than the iterable, *fillvalue* is used in place
    of missing values:

        >>> list(windowed([1, 2, 3], 4))
        [(1, 2, 3, None)]

    Each window will advance in increments of *step*:

        >>> list(windowed([1, 2, 3, 4, 5, 6], 3, fillvalue='!', step=2))
        [(1, 2, 3), (3, 4, 5), (5, 6, '!')]

    To slide into the iterable's items, use :func:`chain` to add filler items
    to the left:

        >>> iterable = [1, 2, 3, 4]
        >>> n = 3
        >>> padding = [None] * (n - 1)
        >>> list(windowed(chain(padding, iterable), 3))
        [(None, None, 1), (None, 1, 2), (1, 2, 3), (2, 3, 4)]
    r