Need Help with constraint for a Facility Location model

Hi,
I am currently working with gams to modell a facility location problem, due to a thesis. sadly i am new to this.
I put the file in the appendix. to put the problem short.

how do i build a constraint, so that at every possible place " i " only one station with size " s " can be opened. ?
so far my approach was

Equation
Constraint only one station can be activated on place i

Constraint…
sum((i,s), akt(i,s)) =l= 1 something like that with akt(i,s) being a binary variable to: 1 if in place i a station with size s is aktivated, 0 else
Facility location Problem with different sizes.gms (2.72 KB)

Hi ToSan

THis would probably be the constraint you are looking for:

OnlyOneI(i)..
    sum(s, akt(i,s)) =l= 1;

At every i there can only be one or zero plants.

Cheers
Renger