{ diff(x1, t) = (1*k7*k4*x6 + (-1)*k7*k2*x3)/k7,
  diff(x3, t) = (2*k7*k3*x2*x6 + (-1)*k7*k2*x3 + (-1)*k7*k1*x4)/k7,
  diff(x4, t) = (1*k7*k2*x3 + (-1)*k7*k1*x4)/k7,
  diff(x5, t) = 0 }