{ diff(x1, t) = (1*k18*k16*k8 + (-1)*k18*k1*x1*x5 + (-1)*k18*k2*x1*x6 + (-1)*k18*k8*x1)/k18,
  diff(x2, t) = (1*k18*k1*x1*x5 + (-1)*k18*k3*x2 + (-1)*k18*k4*x2*x7)/k18,
  diff(x3, t) = (1*k18*k3*x2 + (-1)*k18*k9*x3 + (-1)*k18*k4*x3*x7)/k18,
  diff(x4, t) = (1*k18*k2*x1*x6 + (-1)*k18*k10*x4)/k18,
  diff(x5, t) = (1*k18*k7*x3 + (-1)*k18*k13*x5)/k18,
  diff(x6, t) = (1*k18*k6*x3 + (-1)*k18*k11*x6)/k18,
  diff(x7, t) = (1*k18*k17*k12 + 1*k18*k5*x3 + (-1)*k18*k12*x7)/k18 }