function (xx, N) 
{
# xx must be a nonempty subset of 1:N
    m = length(xx)
    n=N-m
    yy=(1:N)[-xx]
    Z =1:N
    
    s = NULL
    for (j in 1:N) {
        s[j] = mean(xx<=Z[j])-mean(yy<=Z[j])
    }
    KS.stat = max(abs(s))
    KS.stat
}