{ diff(x1, t) = ((-1)*k4*k5*x3*x1 + (-1)*k4*k7*x4*x1 + (-1)*k4*k2*x1 + 1*k4*k12)/k4,
  diff(x2, t) = (1*k4*k5*x3*x1 + 1*k4*k7*x4*x1 + (-1)*k4*k1*x2 + (-1)*k4*(k8*x2*k14 - k9*k15) + 1*k4*k10*k15)/k4,
  diff(x3, t) = ((-1)*k4*k6*x2^k3*x3 + (-1)*k4*k1*x3 + 1*k4*k11)/k4,
  diff(x4, t) = (1*k4*k6*x2^k3*x3 + (-1)*k4*k1*x4)/k4,
  diff(x5, t) = 0,
  diff(x6, t) = 0 }