John,
Not sure what this is about, but I'll give it a try anyway ...
Does the following yield the result you want?
aa(x) := x^2; /* you can omit the block([], ...) */
d : concat ('a, 'a); /* concat('a, ...) => symbol, concat("a", ...)
=> string */
=> aa
d(2);
=> 4
Hope this helps
Robert Dodier