The SRFI document says:

  bag-product n bag
  bag-product! n bag

But the reference implementation and its test has
the arguments reversed:

  bag-product bag n
  bag-product! bag n

I guess at some point the arg order was reversed in the doc,
probably to keep it consistent with *-map and *-for-each.
Anyway, now that the srfi is finalized, shall we fix the
reference implementation?

--shiro