{{alias}}( W )
    Returns an accumulator function which incrementally computes a moving root
    mean squared error (RMSE).

    The `W` parameter defines the number of values over which to compute the
    moving root mean squared error.

    If provided a value, the accumulator function returns an updated moving root
    mean squared error. If not provided a value, the accumulator function
    returns the current moving root mean squared error.

    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.

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

    Examples
    --------
    > var accumulator = {{alias}}( 3 );
    > var r = accumulator()
    null
    > r = accumulator( 2.0, 3.0 )
    1.0
    > r = accumulator( -5.0, 2.0 )
    5.0
    > r = accumulator( 3.0, 2.0 )
    ~4.12
    > r = accumulator( 5.0, -2.0 )
    ~5.74
    > r = accumulator()
    ~5.74

    See Also
    --------