[ Eq(x1, k7 + Piecewise((k8, Le(t, k11)), (0, True)) + k9*t/k12),
  Eq(1*k13*x4 + 1*k13*x5 + 1*k13*x6 + 1*k13*x8, k13*k27),
  Eq(1*k13*x7 + 1*k13*x8 + 1*k13*x9, k13*k28) ]