{{alias}}( W[, mean] )
    Returns an accumulator function which incrementally computes a moving
    unbiased sample variance.

    The `W` parameter defines the number of values over which to compute the
    moving unbiased sample variance.

    If provided a value, the accumulator function returns an updated moving
    unbiased sample variance. If not provided a value, the accumulator function
    returns the current moving unbiased sample variance.

    As `W` values are needed to fill the window buffer, the first `W-1` returned
    values are calculated from smaller sample sizes. Until the window is full,
    each returned value is calculated from all provided values.

    Parameters
    ----------
    W: integer
        Window size.

    mean: number (optional)
        Known mean.

    Returns
    -------
    acc: Function
        Accumulator function.

    Examples
    --------
    > var accumulator = {{alias}}( 3 );
    > var s2 = accumulator()
    null
    > s2 = accumulator( 2.0 )
    0.0
    > s2 = accumulator( -5.0 )
    24.5
    > s2 = accumulator( 3.0 )
    19.0
    > s2 = accumulator( 5.0 )
    28.0
    > s2 = accumulator()
    28.0

    See Also
    --------