[racket] reference #:when clause in for/list?

Hi Mark

'in-value' -- which is new to me - thanks Sam! -- is cleaner than using a
'let' (or 'define') outside the for/list, hence preferable.

Re-factoring my earlier example:

(define (mersenne-primes-3 n)
  (for*/list ([i n]
              [c (in-value (sub1 (expt 2 i)))]
              #:when (prime? c))
    (list i c)))


Core Racketeers: Maybe add an 'in-values' example to the docs?

Dan
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