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

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

    If provided `NaN` or a value which, when used in computations, results in
    `NaN`, the accumulated value is `NaN` for all future invocations.

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

    Examples
    --------
    > var accumulator = {{alias}}();
    > var m = accumulator()
    null
    > m = accumulator( 2.0, 3.0 )
    ~33.33
    > m = accumulator( 5.0, 2.0 )
    ~-58.33
    > m = accumulator()
    ~-58.33

    See Also
    --------