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

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

    If provided a value, the accumulator function returns updated accumulated
    values. If not provided a value, the accumulator function returns the
    current moving accumulated values.

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

    Parameters
    ----------
    out: Array|TypedArray (optional)
        Output array.

    W: integer
        Window size.

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

    Examples
    --------
    > var accumulator = {{alias}}( 3 );
    > var v = accumulator()
    null
    > v = accumulator( 2.0 )
    [ 2.0, 0.0 ]
    > v = accumulator( -5.0 )
    [ -1.5, 24.5 ]
    > v = accumulator( 3.0 )
    [ 0.0, 19.0 ]
    > v = accumulator( 5.0 )
    [ 1.0, 28.0 ]
    > v = accumulator()
    [ 1.0, 28.0 ]

    See Also
    --------