{ diff(x1, t) = 0,
  diff(x2, t) = (1*k16*(k1 + k2*x6/(k3 + x6)) + (-1)*k16*k4*x2 + (-1)*k16*(k5*k17*x2 - k6*x3))/k16,
  diff(x3, t) = (1*k16*(k5*k17*x2 - k6*x3) + (-1)*k16*k7*x3 + (-1)*k16*(k8*x3*x4 - k9*x5) + 1*k16*k10*x5)/k16,
  diff(x4, t) = ((-1)*k16*(k8*x3*x4 - k9*x5) + 1*k16*(k13 + k14*x6/(k3 + x6)) + (-1)*k16*k15*x4)/k16,
  diff(x5, t) = (1*k16*(k8*x3*x4 - k9*x5) + (-1)*k16*k10*x5 + (-1)*k16*k11*x5)/k16,
  diff(x6, t) = (1*k16*k10*x5 + (-1)*k16*k12*x6)/k16 }