induction c; intuition; simpl in *.
inversion H; clear H; subst.
exists O.
simpl.
trivial.
destruct (evalDet_step c s); discriminate.
destruct (shiftOut s n).
destruct p.
discriminate.
discriminate.
discriminate.
Qed.

(* Specifications: *)
Lemma skipn_sum : forall (A : Type)(n2 n1 : nat)(ls : list A),
skipn n1 (skipn n2 ls) = skipn (n2 + n1) ls.