End EXTEQ.
Section EXTEXT.

Lemma extext: forall
(SimExt12 : Forward_simulation_extends Sem1 Sem2 g1 g2 epts12)
(SimExt23 : Forward_simulation_extends Sem2 Sem3 g2 g3 epts23),
Forward_simulation_extends Sem1 Sem3 g1 g3 epts13.