{ diff(x1, t) = (1*k12*k4*k1 + (-1)*k12*k4*x1 + (-1)*k12*k3*x1*x4)/k12,
  diff(x2, t) = ((-1)*k12*k5*x2 + 1*k12*k3*x1*x4 + (-1)*k12*k9*x3*x2)/k12,
  diff(x3, t) = ((-1)*k12*k10*x3 + 1*k12*k11*x1*x4*x3/(k1 + x3))/k12,
  diff(x4, t) = ((-1)*k12*k6*x4 + 1*k12*k5*k2*x2)/k12 }