induction c; intuition; simpl in *.
inversion H; clear H; subst.
trivial.
destruct (evalDet_step c s1); discriminate.
destruct (shiftOut s1 n); [ destruct p ; discriminate | discriminate ].
discriminate.
Qed.

Lemma shiftOut_skipn : forall n (v : Bvector n) s s',
shiftOut s n = Some (v, s') ->
s' = skipn n s.