{ diff(x1, t) = (1*k6*k5 + (-1)*k6*(k2*x1*x2 + k3*x1))/k6,
  diff(x2, t) = (1*k6*(k2*x1*x2 + k4*x3) + (-1)*k6*k1*x1*x2)/k6,
  diff(x3, t) = (1*k6*(k1*x1*x2 + k3*x1) + (-1)*k6*k4*x3)/k6 }