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

    The `W` parameter defines the number of values over which to compute the
    moving unbiased sample covariance.

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

    As `W` (x,y) 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.

    mx: number (optional)
        Known mean.

    my: number (optional)
        Known mean.

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

    Examples
    --------
    > var accumulator = {{alias}}( 3 );
    > var v = accumulator()
    null
    > v = accumulator( 2.0, 1.0 )
    0.0
    > v = accumulator( -5.0, 3.14 )
    ~-7.49
    > v = accumulator( 3.0, -1.0 )
    -8.35
    > v = accumulator( 5.0, -9.5 )
    -29.42
    > v = accumulator()
    -29.42

    See Also
    --------