{ diff(x1, t) = ((-1)*(k1*k16*x1 + k2*x1) + 1*k3*x2 + 1*k8*x3 + 1*k9*x5)/k20,
  diff(x2, t) = (1*(k1*k16*x1 + k2*x1) + (-1)*k3*x2 + (-1)*k4*x2)/k20,
  diff(x3, t) = (1*k4*x2 + (-1)*k5*x3 + (-1)*k8*x3)/k20,
  diff(x4, t) = (1*k5*x3 + (-1)*x4*(k6 + k7*x9/(1 + x9)))/k20,
  diff(x5, t) = (1*x4*(k6 + k7*x9/(1 + x9)) + (-1)*k9*x5)/k20,
  diff(x6, t) = ((-1)*k10*x6*(x3 + k11*x4)/(1 + k13*x9) + 1*k12*x7)/k20,
  diff(x7, t) = (1*k10*x6*(x3 + k11*x4)/(1 + k13*x9) + (-1)*k12*x7)/k20,
  diff(x8, t) = ((-1)*k14*x8*x7 + 1*k15*x9)/k20,
  diff(x9, t) = (1*k14*x8*x7 + (-1)*k15*x9)/k20 }