I've pushed new versions of SRFI 113 and sets.tar.gz
to <http://www.ccil.org/~cowan/temp/srfi-113.html> and
<http://www.ccil.org/~cowan/temp/sets.tar.gz>.

1) Eliminate zero-argument versions of the comparisons.  Previously,
(set=?) would return #t; now it is an error.

2) Add a note that it's an error to add or delete an element from a set
or bag while iterating over it.

Implementation changes:

1) Remove support for zero-argument comparisons, per above.  I was
writing a comment justifying it, and decided it was silly to allow it.

2) Add unique code for set-xor! and bag-xor!, which previously just
invoked set-xor and bag-xor.  This was valid by the contract for
linear-update procedures, but Sam Tobin-Hochstadt pointed out how I
could implement a proper bag-xor!.  (Moral: don't assume you should
always process the first argument first.)

3) Added lots of comments to sets-impl.scm explaining the various
algorithms used.

All tests still pass.

--
John Cowan          http://www.ccil.org/~cowan        xxxxxx@ccil.org
Economists were put on this planet to make astrologers look good.
        --Leo McGarry