Section Undup.
Variable X : eqType.
Implicit Types A B : list X.
Fixpoint undup A : list X :=
match A with
| nil => nil
| x::A' => if decision (x el A') then undup A' else  x :: undup A'
end.

Lemma rep_dupfree A U :
dupfree U -> A el power U -> rep A U = A.