{ x1 = k7 + piecewise(k8, t <= k11, 0) + k9*t/k12,
  1*k13*x4 + 1*k13*x5 + 1*k13*x6 + 1*k13*x8 = k13*k27,
  1*k13*x7 + 1*k13*x8 + 1*k13*x9 = k13*k28 }