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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?
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.