{ diff(x2, t) = k5*x1 + k6*k7*x2*(1 - k8*(x2 + x3)) - k10*x2,
  diff(x3, t) = (1 - k5)*x1 + k6*k7*x3*(1 - k8*(x2 + x3)) - k10*x3,
  diff(x4, t) = k9*x2 - k11*x4 }