{{alias}}( W )
    Returns an accumulator function which incrementally computes a moving
    mean percentage error (MPE).

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

    If provided input values, the accumulator function returns an updated moving
    mean percentage error. If not provided input values, the accumulator
    function returns the current moving mean percentage error.

    As `W` (f,a) 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 m = accumulator()
    null
    > m = accumulator( 2.0, 3.0 )
    ~33.33
    > m = accumulator( 5.0, 2.0 )
    ~-58.33
    > m = accumulator( 3.0, 2.0 )
    ~-55.56
    > m = accumulator( 2.0, 5.0 )
    ~-46.67
    > m = accumulator()
    ~-46.67

    See Also
    --------