intuition.
assert (l = skipn 0 l).
simpl.
trivial.
rewrite H1.
eapply list_skipn_strong_ind_h; trivial.
Qed.

Lemma evalDet_step_nil_inv : forall (A : Set)(c : Comp A)(a1 a2 : A) s2,
evalDet_step c nil = (cs_done a2 s2) ->
In a1 (getSupport c) ->
a1 = a2.