lingo报错 index variables may not share names with other varia

lingo报错 index variables may not share names with other variables
sets:
teacher/1..8/;
lunshu/1..5/;
link(teacher,lunshu):x;
endsets
min=@sum(link(i,j):x(i,j)*0.5);
@bnd(18,x(i,j),19);
@for(lunshu(j):@sum(teacher(i):x(i,j))=a(j););
@gin(link(i,j):x(i,j));
一直不停地报错!