End EXTEXT.
Section EXTINJ.

Lemma extinj: forall
(SimExt12 : Forward_simulation_extends Sem1 Sem2 g1 g2 epts12)
(SimInj23 : Forward_simulation_inject Sem2 Sem3 g2 g3 epts23),
Forward_simulation_inject Sem1 Sem3 g1 g3 epts13.