(define (breadth . trees) (or trees (cons (map car trees) (apply breadth (apply append (map cdr trees)))) '()))
but the perhaps-valid question you could then ask is: why isn't '() coerced to #f, and conversely why there isn't a coerce-to-list form for booleans. dunno. MIT scheme treats them the same, so perhaps they didn't see the logic in it either.