"Linear update" APIs make me very uncomfortable and nervous.
Names like mapping-set! and mapping-replace! are especially dangerous,
and most definitely do not belong in a programming-language standard as
would be used by typical textbooks and programmers, at least without
compiler support.  It's just guaranteed to lead to unportable programs
and hard-to-find bugs.

If there were some way to annotate a procedure with a "result-must-be-used"
property then one might consider the idea, but as it is: don't.

The procedure mapping-replace! must be *guaranteed* to update-in-place
its first parameter, and should have unspecified result.

The linear-update versions should be moved to a separate library, and
renamed to something line mapping-linear-replace.

--
	--Per Bothner
xxxxxx@bothner.com   http://per.bothner.com/