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