Thanks Nicolas, that's also what I thought 
Now we try to check the cart content when the customer clicks the "Panier" button.
This is only valid if the customer comes form a "A la carte" link or sublink :
We have to be sure that the customer does not remove a product from Etape 1 if he has a product from Etape 2 in the cart.
If he does so we want him to be redirected to the A la carte main menu entry and it's cart emptied.
Have you maybe an idea how we could reach this ?
Thanks in advance for your help and advise,
PS : the "A la Carte" menu control works with the Regular Labs ReReplace extension !
Best regards
Stéphane