Definition ev_step_det:
        forall (m m' m'' : mem) (c c' c'' : semC) ev ev',
          ev_step semSem c m ev c' m' ->
          ev_step semSem c m ev' c'' m'' -> c' = c'' /\ m' = m'' /\ ev = ev'.

Lemma ev_step_decay: forall c c' m m' ev, ev_step semSem c m ev c' m' -> decay m m'.