intuition.
edestruct (@evalDet_steps_dec _ c).
trivial.
destruct H0.
destruct H0.
left.
econstructor.
econstructor.
eauto.
right.
econstructor.
eauto.
Qed.

Lemma evalDet_step_app_done_eq : forall (A : Set)(c : Comp A) s s' s'' a,
evalDet_step c s = (cs_done a s'') ->
evalDet_step c (s ++ s') = (cs_done a (s'' ++ s')).