write sum in gams not for all member of set

Question

i want to write sum in gams like this: sets s/A,B,C/ R(machinery,food) . . . machinery_product..sum(Destination,x(machinery,A,Destination))=e=0

but program give me error because A and machinery arent set and they are member of set how can i write this

Answer 1

Set elements need to be quoted when used as index. I.e. for set s with element A: x(s) and x('A').