intuition.
destruct a1.
destruct a2.
rewrite <- rndListElem_support in *.
unfold rndListElem.
case_eq (length ls); intuition.
destruct ls; simpl in *. intuition. omega.
eapply comp_spec_impl_eq.
eapply comp_spec_seq.
apply (Some a).
apply (Some a).
eapply eq_impl_comp_spec.
eapply well_formed_RndNat.
omega.
eapply well_formed_RndNat.
omega.
eapply RndNat_uniform.
3:{
intros.
simpl in H5.
eapply comp_spec_ret.
assert (a1 = (firstIndexOf (EqDec_dec _) ls a 0) <->
b = (firstIndexOf (EqDec_dec _) ls a0 0)).
eapply H5.
clear H5.
intuition; subst.
rewrite H5.
eapply nth_firstIndexOf; trivial.
symmetry.
eapply nth_firstIndexOf_if; intuition.
rewrite H7.
eapply nth_firstIndexOf; trivial.
symmetry.
eapply nth_firstIndexOf_if; intuition.
}
rewrite <- H2.
apply firstIndexOf_in_lt; trivial.
rewrite <- H2.
apply firstIndexOf_in_lt; trivial.
apply rndListElem_support in H0.
apply rndListElem_support_None in H1.
subst.
simpl in *.
intuition.
destruct a2.
apply rndListElem_support in H1.
apply rndListElem_support_None in H0.
subst.
simpl in *.
intuition.
intuition.
Qed.

Lemma not_in_nth_option :
forall (A : Set)(ls : list A)(a : A)(i : nat),
(~In a ls) ->
nth_option ls i = Some a ->
False.