Unfortunately, the implementation is buggy and makes the last appearance
win. That's easy to fix for alist->bag, but tricky for alist->bag!,
where we want the first appearance *in the alist* to win, beating both
later appearances and any existing entry. Suggestions?

How about something like this (untested pseudocode) for alist->bag!:

(define (alist->bag! bag alist)

(bag-delete-all! bag (map car alist))

(for-each (lambda (pair)

(unless (bag-contains? bag (car pair))

(bag-increment! bag (car pair) (cdr pair))))

alist))

If the other bag operations are efficient then this algorithm should be optimal asymptotically, if not up to constant factors.

Kevin Wortman