{ diff(x1, t) = ((-1)*k20*x1*x13 + 1*k21*x2 + 1*k25*x5)/k1,
  diff(x2, t) = (1*k20*x1*x13 + (-1)*k21*x2 + (-1)*k22*x2)/k1,
  diff(x3, t) = (1*k22*x2 + (-1)*k23*x3*x4 + 1*k24*x5 + (-1)*k26*x3*x13 + 1*k27*x6 + 1*k31*x8)/k1,
  diff(x4, t) = ((-1)*k23*x3*x4 + 1*k24*x5 + 1*k25*x5 + (-1)*k29*x4*x7 + 1*k30*x8 + 1*k31*x8)/k1,
  diff(x5, t) = (1*k23*x3*x4 + (-1)*k24*x5 + (-1)*k25*x5)/k1,
  diff(x6, t) = (1*k26*x3*x13 + (-1)*k27*x6 + (-1)*k28*x6)/k1,
  diff(x7, t) = (1*k28*x6 + (-1)*k29*x4*x7 + 1*k30*x8)/k1,
  diff(x8, t) = (1*k29*x4*x7 + (-1)*k30*x8 + (-1)*k31*x8)/k1,
  diff(x9, t) = ((-1)*k8*x9*x19 + 1*k9*x16 + 1*k13*x12)/k1,
  diff(x10, t) = (1*k10*x16 + (-1)*k11*x10*x11 + 1*k12*x12 + (-1)*k14*x10*x19 + 1*k15*x15 + 1*k19*x14)/k1,
  diff(x11, t) = ((-1)*k11*x10*x11 + 1*k12*x12 + 1*k13*x12 + (-1)*k17*x11*x13 + 1*k18*x14 + 1*k19*x14)/k1,
  diff(x12, t) = (1*k11*x10*x11 + (-1)*k12*x12 + (-1)*k13*x12)/k1,
  diff(x13, t) = (1*k16*x15 + (-1)*k17*x11*x13 + 1*k18*x14 + (-1)*k20*x1*x13 + 1*k21*x2 + 1*k22*x2 + (-1)*k26*x3*x13 + 1*k27*x6 + 1*k28*x6)/k1,
  diff(x14, t) = (1*k17*x11*x13 + (-1)*k18*x14 + (-1)*k19*x14)/k1,
  diff(x15, t) = (1*k14*x10*x19 + (-1)*k15*x15 + (-1)*k16*x15)/k1,
  diff(x16, t) = (1*k8*x9*x19 + (-1)*k9*x16 + (-1)*k10*x16)/k1,
  diff(x17, t) = ((-1)*k2*x17*x18 + 1*k3*x22 + 1*k7*x21)/k1,
  diff(x18, t) = ((-1)*k2*x17*x18 + 1*k3*x22 + 1*k4*x22)/k1,
  diff(x19, t) = (1*k4*x22 + (-1)*k5*x19*x20 + 1*k6*x21 + (-1)*k8*x9*x19 + 1*k9*x16 + 1*k10*x16 + (-1)*k14*x10*x19 + 1*k15*x15 + 1*k16*x15)/k1,
  diff(x20, t) = ((-1)*k5*x19*x20 + 1*k6*x21 + 1*k7*x21)/k1,
  diff(x21, t) = (1*k5*x19*x20 + (-1)*k6*x21 + (-1)*k7*x21)/k1,
  diff(x22, t) = (1*k2*x17*x18 + (-1)*k3*x22 + (-1)*k4*x22)/k1 }