{ diff(x1, t) = (1*k21*k2*x2 + (-1)*k3*x1*k24)/k21,
  diff(x2, t) = ((-1)*k21*k2*x2 + 1*k3*x1*k24 + (-1)*k21*k4*x2 + 1*k21*k5*x3 + 1*k21*k14*x4)/k21,
  diff(x3, t) = (1*k21*k4*x2 + (-1)*k21*k5*x3 + (-1)*k21*k13*x3*x11)/k21,
  diff(x4, t) = (1*k21*k13*x3*x11 + (-1)*k21*k14*x4)/k21,
  diff(x5, t) = ((-1)*k21*k6*x5*x3 + (-1)*k7*x5 + 1*k8*x12)/k21,
  diff(x6, t) = (1*k21*k6*x5*x3 + (-2)*k21*k9*x6^2 + 2*k21*k10*x8 + (-1)*k21*k9*x6*x7 + 1*k21*k10*x9 + (-1)*k7*x6 + 1*k8*x14)/k21,
  diff(x7, t) = ((-1)*k21*k9*x6*x7 + 1*k21*k10*x9 + (-1)*k7*x7 + 1*k8*x16)/k21,
  diff(x8, t) = (1*k21*k9*x6^2 + (-1)*k21*k10*x8 + (-1)*k11*k7*x8)/k21,
  diff(x9, t) = (1*k21*k9*x6*x7 + (-1)*k21*k10*x9 + (-1)*k11*k7*x9)/k21,
  diff(x10, t) = (1*k17*x17 + (-1)*k21*k18*x10)/k21,
  diff(x11, t) = ((-1)*k21*k13*x3*x11 + 1*k21*k14*x4 + 1*k21*k19*x10 + (-1)*k21*k20*x11)/k21,
  diff(x12, t) = (1*k7*x5 + (-1)*k8*x12 + 1*k22*k12*x14)/k22,
  diff(x13, t) = (1*k11*k7*x8 + 1*k22*k9*x14^2 + (-1)*k22*k10*x13)/k22,
  diff(x14, t) = (1*k7*x6 + (-1)*k8*x14 + (-1)*k22*k12*x14 + (-2)*k22*k9*x14^2 + 2*k22*k10*x13 + (-1)*k22*k9*x14*x16 + 1*k22*k10*x15)/k22,
  diff(x15, t) = (1*k11*k7*x9 + 1*k22*k9*x14*x16 + (-1)*k22*k10*x15)/k22,
  diff(x16, t) = (1*k7*x7 + (-1)*k8*x16 + (-1)*k22*k9*x14*x16 + 1*k22*k10*x15)/k22,
  diff(x17, t) = (1*k22*k15*x15^k1/(x15^k1 + k16^k1) + (-1)*k17*x17)/k22,
  diff(x18, t) = 0 }