{ diff(x1, t) = (1*k1*x2 + (-1)*k2*x1*x3)/k15,
  diff(x2, t) = (1*k13*x7 + (-1)*k14*x2)/k15,
  diff(x3, t) = (1*k3*x4 + (-1)*k4*x3)/k15,
  diff(x4, t) = (1*k9*x1 + (-1)*k10*x6*x4)/k15,
  diff(x5, t) = (1*k5*x2 + (-1)*k6*x3*x5)/k15,
  diff(x6, t) = (1*k7 + (-1)*k8*x5*x6)/k15,
  diff(x7, t) = (1*k11*x1 + (-1)*k12*x7)/k15 }