xxtoss = {x1 -> s1, x2 -> s1, x3 -> s1, x4 -> s2, x5 -> s2, x6 -> s2, 
     x7 -> s3, x8 -> s3, x9 -> s3, x10 -> s3, x11 -> s3, x12 -> s3, 
     x13 -> s4, x14 -> s4, x15 -> s4}