{ diff(x1, t) = (-1)*k18*k1*x1*x16/k18,
  diff(x2, t) = (1*k18*k1*x1*x16 + (-1)*k18*k2*x2*x8 + (-1)*k18*k3*x2*x17 + (-1)*k18*k4*x2*x18)/k18,
  diff(x3, t) = (1*k18*k2*x2*x8 + (-1)*k18*k5*x3*x8 + (-1)*k18*k6*x3*x17 + (-1)*k18*k7*x3*x18)/k18,
  diff(x4, t) = (1*k18*k3*x2*x17 + (-1)*k18*k5*x4*x8 + (-1)*k18*k6*x4*x17 + (-1)*k18*k7*x4*x18)/k18,
  diff(x5, t) = (1*k18*k4*x2*x18 + (-1)*k18*k5*x5*x8 + (-1)*k18*k6*x5*x17 + (-1)*k18*k7*x5*x18)/k18,
  diff(x6, t) = (1*k18*k5*x3*x8 + (-1)*k18*k8*x6*x6 + 1*k18*k10*x8*x10)/k18,
  diff(x7, t) = (-1)*k18*k9*x7*x9/k18,
  diff(x8, t) = ((-1)*k18*k2*x2*x8 + (-1)*k18*k5*x3*x8 + (-1)*k18*k5*x4*x8 + (-1)*k18*k5*x5*x8 + (-1)*k18*k10*x8*x10)/k18,
  diff(x9, t) = (1*k18*k8*x6*x6 + (-1)*k18*k11*x9)/k18,
  diff(x10, t) = (1*k18*k9*x7*x9 + (-1)*k18*k12*x10)/k18,
  diff(x11, t) = (1*k18*k6*x3*x17 + 1*k18*k5*x4*x8 + (-1)*k18*k13*x11*x19)/k18,
  diff(x12, t) = (-1)*k18*k14*x12*x14/k18,
  diff(x13, t) = (1*k18*k14*x12*x14 + (-1)*k18*k15*x13)/k18,
  diff(x14, t) = (1*k18*k13*x11*x19 + (-1)*k18*k16*x14)/k18,
  diff(x15, t) = (1*k18*k15*x13 + (-1)*k18*k17*x15)/k18,
  diff(x16, t) = (-1)*k18*k1*x1*x16/k18,
  diff(x17, t) = ((-1)*k18*k3*x2*x17 + (-1)*k18*k6*x3*x17 + (-1)*k18*k6*x4*x17 + (-1)*k18*k6*x5*x17)/k18,
  diff(x18, t) = ((-1)*k18*k4*x2*x18 + (-1)*k18*k7*x3*x18 + (-1)*k18*k7*x4*x18 + (-1)*k18*k7*x5*x18)/k18,
  diff(x19, t) = (-1)*k18*k13*x11*x19/k18,
  diff(x20, t) = (1*k18*k7*x3*x18 + 1*k18*k5*x5*x8)/k18,
  diff(x21, t) = 1*k18*k6*x4*x17/k18,
  diff(x22, t) = (1*k18*k7*x4*x18 + 1*k18*k6*x5*x17)/k18,
  diff(x23, t) = 1*k18*k7*x5*x18/k18 }