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