{ diff(x1, t) = (1*k9*k1*x1 + (-1)*k9*k1*x1*x1 + (-1)*k9*k1*x1*x2 + (-1)*k9*k2*x1*x3)/k9,
  diff(x2, t) = ((-1)*k9*k3*x2*x4 + (-1)*k9*x2 + 1*k9*k2*x1*x3)/k9,
  diff(x3, t) = (1*k9*k8*x2 + (-1)*k9*k5*x3 + (-1)*k9*k4*x3*x4 + (-1)*k9*k2*x1*x3)/k9,
  diff(x4, t) = (1*k9*k6*x2*x4 + (-1)*k9*k7*x4)/k9 }