{ diff(x1, t) = ((-1)*k1*x1*k2 + 1*k3*x2*k1)/k1,
  diff(x2, t) = (1*k1*x1*k2 + (-1)*k3*x2*k1 + (-1)*k1*k13*x2)/k1,
  diff(x3, t) = ((-1)*x3*k4*x2*k1 + 1*k5*x4*k1)/k1,
  diff(x4, t) = (1*x3*k4*x2*k1 + (-1)*k5*x4*k1)/k1,
  diff(x5, t) = (-1)*x5*k6*x4*k1/k1,
  diff(x6, t) = (1*x5*k6*x4*k1 + (-1)*x6*k7*x7*k1)/k1,
  diff(x7, t) = (-1)*k8*x7*k14*k1/k1,
  diff(x8, t) = (1*x6*k7*x7*k1 + 1*k1*k9/(1 + x11^3) + (-1)*k11*x8*k1)/k1,
  diff(x9, t) = 0,
  diff(x10, t) = 1*k8*x7*k14*k1/k1,
  diff(x11, t) = (1*k1*k10/(1 + x8^3) + (-1)*k1*x11*k12)/k1,
  diff(x12, t) = 1*k1*k13*x2/k1 }