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