{{alias}}( [mean] )
    Returns an accumulator function which incrementally computes a variance-to-
    mean ratio (VMR).

    If provided a value, the accumulator function returns an updated accumulated
    value. If not provided a value, the accumulator function returns the current
    accumulated value.

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

    Parameters
    ----------
    mean: number (optional)
        Known mean.

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

    Examples
    --------
    > var accumulator = {{alias}}();
    > var D = accumulator()
    null
    > D = accumulator( 2.0 )
    0.0
    > D = accumulator( 1.0 )
    ~0.33
    > D = accumulator()
    ~0.33

    See Also
    --------