Open:
/Coq/Sets/Powerset_facts/Sets_as_an_algebra/Union_commutative.con

We prove the conjunction again:

alias Ensemble /Coq/Sets/Ensembles/Ensembles/Ensemble.con
alias Union    /Coq/Sets/Ensembles/Ensembles/Union.ind#1/1
alias Included /Coq/Sets/Ensembles/Ensembles/Included.con
alias and      /Coq/Init/Logic/Conjunction/and.ind#1/1

The two parts of the conjunction can be proved in the same way. So we
can make a Cut:

!V:Set.!C:(Ensemble V).!D:(Ensemble V).(Included V (Union V C D)
(Union V D C))