Thanks Georges, Matthias, and Philip for the further pointers:
Helped by your comments I inferred that I could construct a legitimate
unsupplied-arg case using case-lambda.
I think that this is the kind of illustration would be helpful in the docs:
(module server racket
[f (->* (number?) (number?) number?)]
[g (->i ([a real?])
#:post (a b result)
(let ([B (if (unsupplied-arg? b) (default-b) b)])
(= (* (sgn a) (sgn B)) (sgn result))))]))
(define (default-b) 10)
(define f (case-lambda
[(a) (+ a (default-b))]
[(a b) (+ a b)]))
(define g (case-lambda
[(a) (* a (default-b))]
[(a b) (* a b)])))
(f 10 -5)
(g 10 -5)
I suppose this falls into the class of learning challenges where certain
aspects of the design of a feature are explained by the existence of
"advanced" concepts that not everyone may be yet using routinely, or even
be familiar with.
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