{ diff(x1, t) = (-1)*k1*k2*x1*x2/k1,
  diff(x2, t) = (-1)*k1*k2*x1*x2/k1,
  diff(x3, t) = (1*k1*k2*x1*x2 + (-1)*k1*k4*x3*x4 + 1*k1*k5*x6)/k1,
  diff(x4, t) = ((-1)*k1*k3*x1*x2*x4 + (-1)*k1*k4*x3*x4)/k1,
  diff(x5, t) = (1*k1*k3*x1*x2*x4 + 1*k1*k6*x3*x7 + (-1)*k1*k8*x5)/k1,
  diff(x6, t) = (1*k1*k4*x3*x4 + (-1)*k1*k5*x6)/k1,
  diff(x7, t) = (1*k1*k5*x6 + (-1)*k1*k6*x3*x7 + (-1)*k1*k7*x7)/k1,
  diff(x8, t) = 1*k1*k7*x7/k1,
  diff(x9, t) = 1*k1*k8*x5/k1 }