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