Hi, I have this code but it says domain violation for set, could someone tell me why? or how do I write the sum x (i, i) that appears in the image thank you very much

sets i assigned station located in the sector /a,b,c,d,e/

j response times in minutes fires in the sector /f,g,h,i,j/;

parameter fe(j) frecuency for emergency (for day)

/

f 2

g 1

h 3

i 1

j 3/;

table a(i,j) average response time to a fire

f g h i j

a 5 12 30 20 15

b 20 4 15 10 25

c 15 20 6 15 12

d 25 15 25 4 10

e 10 25 15 12 5;

binary variable x(i,j) if tract j is assigned to station located in tract i zero in otherwise;

free variable z variable for funtion objective;

equations

funobj objetive funtion response time

rtype1(j) restriction type 1

rtype2(i) restriction type 2

rtype3(i,j) restriction type 3;

funobj… z=e= sum((i,j),a(i,j)*fe(j));

rtype1(j)… sum(i, x(i,i))=e= 2;

rtype2(i)… sum(j, x(i,j))=e= 1;

rtype3(i,j)… x(i,j)=l=x(i,i);

model example /all/;

solve example minimizing z using MIP;