k1 - k2*x1*x2 - k4*x1*x4 - k5*x1*x3 
-x2*(k13 + k2*x1) 
-x3*(k7*k21 + k5*x1 + k6*x2) 
k14*x7 - k15*x4 - 2*k16*x4^2 - k4*x1*x4 
2*k5*x1*x3 - k10*x5 - k8*x5 + 2*k6*x2*x3 
k15*x4 - k11*x6 + k4*x1*x4 
k10*x5 + k15*x4 - k14*x7 
