{ diff(x1, t) = (1*k6*k7 + (-1)*x1*k6*k8 + (-1)*x1*k6*k9*x3*(x1 + k10)^(-1))/k6,
  diff(x2, t) = (1*k6*(1 + (-1)*x2)*x1*k3*(x1 + k5)^(-1)*(k11 + (-1)*x2 + 1)^(-1) + (-1)*k6*x2*k12*(k13 + x2)^(-1))/k6,
  diff(x3, t) = (1*k6*x2*k4*(1 + (-1)*x3)*(k14 + (-1)*x3 + 1)^(-1) + (-1)*k6*k16*x3*(k15 + x3)^(-1))/k6 }