AGGREGATION FUNCTION covariances2(x, y, w){
    xmeans := 0.;
    ymeans := 0.;
    l := _builtin_len;
    if (w > l)
        w := l;
    elif (w > l + 2)
    {
        l := 3;
        w := 4;
    }
    elif(w < 99){
        l := 8;
    }
    elif(w<999)
        w := 6;
    else
        l := l / 2;

    for (i := 0, j:= 0; i < w; i := i+1) {
        xmeans += x[i]; 
        ymeans += y[i]; 
        _builtin_ret[i] := avg (( x(l-w, l) - xmeans ) * (y(l - w, l) - ymeans ));
    }
    for (i := 0; i < l; i += 1) 
    {
        xmeans += (x[i] - x[i - w]) / w;
        ymeans += (y[i] - y[i - w]) / w;   
        _builtin_ret[i] := avg (( x(l-w, l) - xmeans ) * (y(l - w, l) - ymeans ));
    }
    Null
}