{ diff(x1, t) = (-1)*k7*k3*x1*x3/x6/k7,
  diff(x2, t) = (1*k7*k3*x1*x3/x6 + (-1)*k7*k4*x2)/k7,
  diff(x3, t) = (1*k7*k4*x2 + (-1)*k7*k5*x3 + (-1)*k7*k6*x3)/k7,
  diff(x4, t) = 1*k7*k5*x3/k7,
  diff(x5, t) = 1*k7*k6*x3/k7,
  diff(x6, t) = 0/k7 }