{ diff(x1, t) = 1*(k3*x3 - k4*k5^k6*x1)/k2,
  diff(x2, t) = 1*(k7*k8^k9*x3 - k10*x2)/k2,
  diff(x3, t) = ((-1)*(k3*x3 - k4*k5^k6*x1) + (-1)*(k7*k8^k9*x3 - k10*x2) + (-1)*(k11*x3 - k12*x4))/k2,
  diff(x4, t) = 1*(k11*x3 - k12*x4)/k2 }