{ diff(x1, t) = (1*k2*k1*x4 + (-1)*k7*x1 + (-1)*k11*x1 + (-1)*k12*x1*x2)/k16,
  diff(x2, t) = (1*k4*x1 + 1*k5*x3*x1 + (-1)*k8*x2)/k16,
  diff(x3, t) = (1*k6*x1 + (-1)*k9*x3)/k16,
  diff(x4, t) = ((-1)*k1*x4 + 1*k3*x3 + (-1)*k10*x4)/k16,
  diff(x5, t) = 0 }