a    b   c
s0 s1 s2    s3
  s0
 s2   s3
s0 a s1
s0 b s0
s0 c s3
s1 a s1
s1 b s2
s1 c s0
s2 a s2
s2 b s1
s2 c s3
s3 a s3
s3 b s2
s3 c s0