Behavior of #!optional

[Théo Cavignac]

Hello everyone,

Quick question: is it safe to rely on the fact that positional 
parameters are available to definition of optional parameters default 
value ?

In other word, is the following procedure OK or does it rely on 
implementation detail that may change ?

(define (f a b #!optional (err (lambda () (error "a is not b" (cons a 
b)))))

 (if (eq? a b) #t (err)))


Thank you !

PS: It's a dumb example but I have a real use case where it would be 
nice to use that trick.