{{alias}}( W )
    Returns an accumulator function which incrementally computes a moving sum of
    products.

    The `W` parameter defines the number of (x,y) pairs over which to compute
    the moving sum of products.

    If provided input values, the accumulator function returns an updated moving
    sum. If not provided input values, the accumulator function returns the
    current moving sum.

    As `W` (x,y) pairs 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 s = accumulator()
    null
    > s = accumulator( 2.0, 3.0 )
    6.0
    > s = accumulator( -5.0, 2.0 )
    -4.0
    > s = accumulator( 3.0, -2.0 )
    -10.0
    > s = accumulator( 5.0, 3.0 )
    -1.0
    > s = accumulator()
    -1.0

    See Also
    --------