{ diff(x1, t) = 0,
  diff(x2, t) = 0,
  diff(x3, t) = 0,
  diff(x4, t) = 0,
  diff(x5, t) = 0,
  diff(x6, t) = 0,
  diff(x7, t) = 0,
  diff(x8, t) = (1*(k1 + k2*k40 + k3*x11 + k4*x11*x10)/(1 + k1/k5 + k6*k40 + k7*x11 + k8*x11*x10) + (-1)*k9*x8 + (-1)*(k17*x8*x9 - k18*x11))/k33,
  diff(x9, t) = ((-1)*(k17*x8*x9 - k18*x11) + 1*(k20 + k21*k40 + k22*x11 + k23*x11*x10)/(1 + k20/k5 + k24*k40 + k25*x11 + k26*x11*x10) + (-1)*k27*x9)/k33,
  diff(x10, t) = (1*(k10 + k11*x11 + k12*x11*x10)/(1 + k10/k5 + k13*x11 + k14*x11*x10 + k15*k39) + (-1)*k16*x10)/k33,
  diff(x11, t) = (1*(k17*x8*x9 - k18*x11) + (-1)*k19*x11)/k33,
  diff(x12, t) = (1*(k28*x11 + k29)/(1 + k29/k5 + k30*x11 + k31*x11*x10) + (-1)*k32*x12)/k33 }