{ diff(x1, t) = (1*k12*(k1 + k2*x4)*x2 + (-1)*k12*k4*x1)/k12,
  diff(x2, t) = ((-1)*k12*(k1 + k2*x4)*x2 + 1*k12*k3*k13)/k12,
  diff(x3, t) = 0 }