[racket] strange interference with prompt, correcture

Thanks,

Jan is right: changing to
(require (except-in racket/control set))
is dispensing with the problem.

Still this is a Racket pitfall
close to be a bug imho.

Btw. there were more slips in my
hastily reduction. In fact, '·indices
should search indices, but therefore the
last argument to 'filter has to be
(range (length L)), not L.

Cheers, Joost