Subset or tuple membership

One may wish to do conditional processing dependent upon whether a set element is defined in a particular subset or tuple.  The form of the syntax that can be employed in the subset case is

Action$subset(setelement)

or

If(subset("quotedsetelement"), Action);

or

While(subset("quotedsetelement"), Action);

or in the tuple case is

Action$tuplename(setelement, set2element)

or

If(tuplename(setelement, set2element), Action);

or

While(tuplename(setelement, set2element), Action);

where in both setelement is a set name which must be controlled within the action or earlier in a Loop statement and quotedsetelement is a particular set element name encased in quotes.

Examples:

(setconditional.gms)

ciZ=sum((allcity,cityj) $cityi(allcity),1);

ciZ$cityi("boston")=sum(allcity,1);

loop((allcity,cityj) $ tuple(allcity,cityj),

      ciz=ciz+ord(allcity)+ord(cityj)*100);

if(cityj("boston"),ciz=1);

if(tuple("orlando","boston"),ciz=1);