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