{ diff(x1, t) = 0,
  diff(x2, t) = (1*k1*k14*k2*k12/k3/(1 + k12/k3 + x2/k4)/(1 + k13/k5) + (-1)*k1*k15*k6*x2/k7/(1 + x2/k7 + x3/k8))/k1,
  diff(x3, t) = (1*k1*k15*k6*x2/k7/(1 + x2/k7 + x3/k8) + (-1)*k1*k16*k9*x3/k10/(1 + x3/k10 + k13/k11))/k1,
  diff(x4, t) = 0,
  diff(x5, t) = 0,
  diff(x6, t) = 0,
  diff(x7, t) = 0 }