2*at + s1 <-> s2 s2 <-> 2*s3 na + s3 <-> at + s4 s4 <-> at + s5 s5 <-> s6 at <-> 0 s3 <-> na s6o <-> 0 s6 <-> (1/10)*s6o s6 <-> na 0 <-> s1