{{alias}}()
    Returns an accumulator function which incrementally computes the mean
    directional accuracy (MDA).

    If provided input values, the accumulator function returns an updated mean
    directional accuracy. If not provided input values, the accumulator function
    returns the current mean directional accuracy.

    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 )
    1.0
    > m = accumulator( -5.0, 4.0 )
    0.5
    > m = accumulator()
    0.5

    See Also
    --------