{ diff(x1, t) = (1*x5*x4 + (-1)*k3*x1)/k8,
  diff(x2, t) = (1*x6*x4 + (-1)*k4*x2)/k9,
  diff(x3, t) = (1*k3*x1 + 1*k4*x2 + 1*k10*k5 + (-1)*k10*k6*x3)/k10,
  diff(x4, t) = ((-1)*x5*x4 + (-1)*x6*x4)/k7 }