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