How can I apply a constraint on all results : (select table) [ xxxxx ] ?

I need to have a constraint but to be applied on multiple results.

number(b.ID) != (select Achats)[number(Atelier.ID) = number(a.Atelier.ID)].Contact.ID

I wish to exclude all contact ID that correspond with the Atelier ID. (I have multiple entry and I wish to exclude all contact associated with the Atelier ID)

if I add first() to the formula it s ok or last...but how to include all ? I have tried with:

let C_1 := first((select Achats)[number(Atelier.ID) = number(a.Atelier.ID)].Contact.ID)

let C_2 := first((select Achats)[number(Atelier.ID) = number(a.Atelier.ID) and number(a.contact.ID) != C_1].Contact.ID)

number(b.ID) != C_1 and number(b.ID) != C_2 but not working...

thanks!