Multiple selection returns value

Hello,
I wish to know the choice in a multiple selection to be done I have created a formula (see below) but it returns me anything. How can I solve this problem

let boite := 'Nature du point';
switch boite do
case 1:
            "1"
case 2:
            "2"
case 3:
            "3"
case 4:
            "4"
case 5:
            "5"
case 6:
            "6"
case 7:
            "7"
case 8:
            "8"
end