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