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'.