Section ShuffleList.
Variable A : Set.
Hypothesis A_EqDec : EqDec A.
Definition shuffle(ls : list A) :=
o <-$ rndListElem _ (getAllPermutations ls);
ret
match o with
| None => nil
| Some x => x
end.
intuition.
unfold shuffle in *.
repeat simp_in_support.
destruct x.
eapply Permutation_sym.
eapply getAllPermutations_perms.
apply rndListElem_support in H0.
trivial.
apply rndListElem_support_None in H0.
exfalso.
eapply getAllPermutations_not_nil.
eauto.
Qed.
Fixpoint permute(ls : list A)(sigma : list nat) : list A :=
match sigma with
| nil => nil
| n :: sigma' =>
match (nth_error ls n) with
| None => nil
| Some a => a :: (permute ls sigma')
end
end.
induction ls; destruct n; intuition; simpl in *.
omega.
omega.
inversion H0.
eapply IHls; eauto.
omega.
Qed.

Theorem permute_length_eq :
forall (sigma : list nat)(ls : list A),
(forall n, In n sigma -> n < length ls) ->
length (permute ls sigma) = length sigma.