{ diff(x1, t) = (1*k1*k2*k13 + (-1)*k1*x1*k13 + (-1)*k3*x2*x1*k13 + 1*k4*x3*k13 + 1*k6*x4*k13)/k12,
  diff(x2, t) = ((-1)*k3*x2*x1*k13 + 1*k4*x3*k13 + 1*k6*x4*k13)/k11,
  diff(x3, t) = (1*k3*x2*x1*k13 + (-1)*k4*x3*k13 + (-1)*k5*x3*k13)/k12,
  diff(x4, t) = (1*k5*x3*k13 + (-1)*k6*x4*k13 + (-1)*k7*x4*k13 + (-1)*k8*x4*k13)/k13,
  diff(x5, t) = 1*k7*x4*k13/k13,
  diff(x6, t) = 1*k8*x4*k13/k11 }