then I get
dak@lola:/usr/local/tmp/guile$ meta/guile /tmp/zorpo.scm 
;;; note: source file /tmp/zorpo.scm
;;;       newer than compiled /usr/local/tmp/guile/cache/guile/ccache/2.2-LE-4-3.4/tmp/zorpo.scm.go
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /tmp/zorpo.scm
;;; compiled /usr/local/tmp/guile/cache/guile/ccache/2.2-LE-4-3.4/tmp/zorpo.scm.go
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.

and what of it?  If you really, really must, do one recursive top-level
check of everything before starting.  But re-verifying structural
integraty after applying every single rule is madness.  Actually,
replacing '+ by 'and in that example will lead to the same bomb-out.  So
it does not look like structural integrity verification is happening
anyway.

> The simplest solution would be to replace all occurrences of "y ..."
> with ". y" in the two macros above, but that has the slight downside
> of making the error message much less comprehensible if you use a
> dotted tail in an 'and' or 'or' form.  Maybe that's not worth worrying
> about though.

If you care about nice error messages, do a single upfront check.

> Alternatively, we could use procedural macros here, but we'd have to
> limit ourselves to very primitive forms in the code because this is so
> early in the bootstrap.

I don't think it's worth it just for and/or (the form generated by or
actually seems more prone to buildup and churn).  But for syntax
replacements in general?  Sure.  You don't want quadratic behavior in
bare-bones replacements like these.

-- 
David Kastrup