set item all items / dish, ink, lipstick, pen, pencil, perfume /
    subitem1(item) first subset / pen, pencil, perfume /
    subitem2(item) second subset;

subitem1('ink') = yes;  subitem1('lipstick') = yes;
subitem2(item) = yes; subitem2('perfume') = no;
display subitem1, subitem2;

sets stuff items sold /pencil, pen/
     firm suppliers /bic, parker, waterman/
     supply(stuff,firm);

supply('pencil','bic') = yes;
supply('pen',firm) = yes;

set allr /n,s,w,e,n-e,s-w/, r(allr);

scalar price /10/;
equations prodbal(allr);
variables activity(allr), revenue(allr);

* equation prodbal declared over allr, defined over r

prodbal(r).. activity(r)*price =e= revenue(r);

* sum over the dynamic set only
parameter totinv, inventory(item);
inventory(item) = ord(item);

totinv = sum(item$subitem1(item), inventory(item));

* alternative for the same thing
totinv = sum(subitem1, inventory(subitem1));

* union
set sunion(item);
sunion(item) = subitem1(item) + subitem2(item); display sunion;

* intersection
set sint(item);
sint(item) = subitem1(item) * subitem2(item); display sint;

* complement
set scomp(item);
scomp(item) = not subitem1(item); display scomp;

* difference
set sdiff(item);
sdiff(item) = subitem1(item) - subitem2(item); display sdiff;