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