RE: Find an element in a functional set (guile-pfds)

>Maybe bbtree-fold can be used, but 
that would not "early exit" as soon as it has found an item, that satisfies some 
specified predicate.

You can use escape continuations to do an early exit:

;; On success, return stuff-to-return. Otherwise, return the symbol ‘no-match’.
(let/ec escape
  (bbtree-fold [...] [... when a match is found, use (escape stuff-to-return)] [...])
  'no-match)

Best regards,
Maxime Devos.