No, it can be anything with a for-each function with the right
signature. A positive integer is a collection, given this function:

(define (integer-for-each proc n)
(let loop ((n n))
(unless (= n 0)
(proc (remainder n 10))
(loop (quotient n 10)))))

I'm not sure how best to explain this. Suggestions, anyone?

OK, I now understand make-for-each-generator's intended behavior, which sounds fine to me, but I think the SRFI text could be better since I didn't understand it until you elaborated.

How about inserting something like the following after the first sentence:

"for-each must be a procedure that, when called as

(for-each proc obj)

calls proc on each element of obj. R7RS for-each, string-for-each, and vector-for-each procedures are examples of appropriate procedures. The values returned by proc are those that are generated by the returned generator."

You're right, it is unclear. Given the existence of a generator
operation for deleting neighbor duplicates, we might as well assume
that there are none.

Sounds good. Yes, if there's a duplicate-filter procedure it's easy enough for a programmer to call it when needed.

Regards,

Kevin