{ diff(x1, t) = ((-1)*k1*x1*x2 + 1*k2*x3 + 1*k3*x3)/k7,
  diff(x2, t) = ((-1)*k1*x1*x2 + 1*k2*x3 + 1*k6*x6)/k7,
  diff(x3, t) = (1*k1*x1*x2 + (-1)*k2*x3 + (-1)*k3*x3)/k7,
  diff(x4, t) = ((-1)*k4*x4*x5 + 1*k5*x6 + 1*k6*x6)/k7,
  diff(x5, t) = (1*k3*x3 + (-1)*k4*x4*x5 + 1*k5*x6)/k7,
  diff(x6, t) = (1*k4*x4*x5 + (-1)*k5*x6 + (-1)*k6*x6)/k7 }