k2*x2 - k1*k18*x1 
k1*k18*x1 - k3*x2 - k2*x2 
k3*x2 - k4*x3 + k5*x4 
k4*x3 - k5*x4 
k7*x6 - k6*k18*x5 
k6*k18*x5 - k8*x6 - k7*x6 
k8*x6 + k10*x8 - k9*k18*x7 
k12*x9 - k11*x8 - k10*x8 + k9*k18*x7 
k11*x8 - k12*x9 
x5 - k21 + x6 + x7 + x8 + x9 
x1 - k22 + x2 + x3 + x4 
