intuition.
unfold G_hybrid, DistSingle_G, B1, B2.
inline_first.
comp_skip.
comp_simp.
comp_swap_r.
comp_skip.
eapply comp_spec_eq_impl_eq.
destruct (lt_dec i (length l)).
eapply comp_spec_eq_trans.
eapply comp_spec_seq_eq; eauto with inhabited.
eapply compMap_unroll.
intuition.
eapply skipn_lt_nil_false;
eauto.
intuition.
eapply comp_spec_eq_refl.
inline_first.
comp_skip.
rewrite hd_skip_eq_nth.
eapply comp_spec_eq_refl.
subst.
inline_first.
rewrite tl_skipn_eq.
comp_skip.
rewrite skipn_gt_nil; intuition.
simpl.
comp_simp.
rewrite app_nil_r.
comp_irr_r.
comp_irr_r.
eapply compMap_wf.
intuition.
Qed.

(* Specifications: *)
Theorem compMap_unroll_tl :
forall (A B : Set)(def : A)(eqdb : EqDec B)(c : A -> Comp B)(ls : list A) n,
length ls = S n ->
comp_spec eq (compMap _ c ls) (y <-$ compMap _ c (firstn n ls); x <-$ c (nth n ls def); ret (y ++ x :: nil)).