{ diff(x3, t) = (1*k15*1/(1 + (k8/x3)^k10)*k4*x3*(logb(k9, 10) - logb(x3, 10))/logb(k9, 10) + (-1)*k15*k5*x1*x3/(k11 + x1 + x3))/k15,
  diff(x1, t) = piecewise(k1*x1*x3/(k3 + x1 + x3), t <= k12, (-(k6 + k2))*x1),
  diff(x2, t) = piecewise(0, t <= k12, k6*x1 - k7*x2) }