{ diff(x1, t) = (1*k7*k1*x1 + (-1)*k7*k1*k2*x1*x1 + (-1)*k7*k3*x1*x2)/k7,
  diff(x2, t) = ((-1)*k7*k6*x2 + 1*k7*k4 + 1*k7*k5*x2*x1)/k7 }