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