[Help-glpk] GLPK cannot compare to symbols: s1 < s2

[Xypron]

Hello Andrew,

I have a symbolic set J, further I want to define a set B identifying
each unordered combination of different elements of aforementioned set:

set J;
set B := setof{j1 in J, j2 in J : j1 < j2} (j1, j2);
display B;
data;
set J := 'A' 'B';

Unfortunately this results in error

Display statement at line 3
test.mod:3: cannot convert A to floating-point number

Is there another way to create set B for a symbolic set J?

If there is not, it would be useful to add some method of comparing
strings to GLPK or to determine the numeric index of an element in a set.

Best regards

Xypron