Цитирую Marc Feeley feeley@iro.umontreal.ca:
(define (square x) (pp x result: (* x x)))
and get as output:
x: 10 result: 100
In Common Lisp I'd turned on tracing for SQUARE, which prints function arguments and results. I'm not much of a Gambit user, so I don't know whether TRACE machinery is in place; if not, it'd be worth-while to add it.
Cheers, Dmitri