{ diff(x1, t) = (1*k15*k11*x5 + (-1)*k15*k12*x1)/k15,
  diff(x2, t) = (1*k16*k9*x5 + (-1)*k16*k10*x2*x4)/k16,
  diff(x3, t) = (1*k16*k3*x2 + (-1)*k16*k4*x3)/k16,
  diff(x4, t) = (1*k16*k7 + (-1)*k16*k8*x4*x7)/k16,
  diff(x5, t) = (1*k16*k1*x7 + (-1)*k16*k2*x5)/k16,
  diff(x6, t) = (1*k16*k13*x1 + (-1)*k16*k14*x6*x3)/k16,
  diff(x7, t) = (1*k16*k5*x6 + (-1)*k16*k6*x7*x3)/k16 }