{{alias}}( [mx, my] )
    Returns an accumulator function which incrementally computes an unbiased
    sample covariance.

    If provided values, the accumulator function returns an updated unbiased
    sample covariance. If not provided values, the accumulator function returns
    the current unbiased sample covariance.

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

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

    my: number (optional)
        Known mean.

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

    Examples
    --------
    > var accumulator = {{alias}}();
    > var v = accumulator()
    null
    > v = accumulator( 2.0, 1.0 )
    0.0
    > v = accumulator( -5.0, 3.14 )
    ~-7.49
    > v = accumulator()
    ~-7.49

    See Also
    --------