tuple Combination_L {
  int labrooms;
  int instructors;
  int studentgroups;
  int labcourses;
};

{Combination_L} L = ...; 
                        
tuple Combination_T {
  int days;
  int slots;
};

{Combination_T} T = ...; 
                      
tuple Combination_L_T{
  int labrooms;
  int days;
  int slots;
  int instructors;
  int studentgroups;
  int labcourses;
};

{Combination_L_T} V_l; 
execute {
 for (var p in L){
   for (var q in T){
     V_l.add(p.labrooms, 
             q.days,
             q.slots, 
             p.instructors, 
             p.studentgroups,
             p.labcourses);
   };
 }; 
};

I want to initialize a derived set Q_l, which represents all possible assignments of room, lecturer, student group and course for the laboratory planning level. These assignments fulfill the condition that the value of the fourth element of the quadruple is l, which means that the lecturer assigned to the quadruple has the value l. These assignments are derived from the variable set V_l
Hier ist my code:

{Combination_L} Q_l;

execute {
    for (var l in instructors) {
        Q_l += {<i, l, m, n> |
         <i, _, _, l, m, n> in V_l};
    };
};

I get a scripting parser error: missing expression.
Please tell me where I made a mistake.
Thanks so much!