{ diff(x1, t) = (1*k12*k1*x1*(1 - (x1 + x2)) + (-1)*k12*k2*x1*x3 + (-1)*k12*(k3*x1*x4 + k4*x1))/k12,
  diff(x2, t) = (1*k12*k2*x1*x3 + (-1)*k12*(k5*x2*x4 + x2))/k12,
  diff(x3, t) = ((-1)*k12*k2*x1*x3 + 1*k12*k11*x2 + (-1)*k12*(k6*x3*x4 + k7*x3))/k12,
  diff(x4, t) = (1*k12*(k8*x2*x4 + k9*x1*x4) + (-1)*k12*k10*x4)/k12 }