Per Bothner scripsit:

> So the place, if any, to mention this warning would seem to be R7RS ....

String-ref and string-set! both have the following remark in R7RS:

    There is no requirement for this procedure to execute in constant time.

--
John Cowan          http://www.ccil.org/~cowan        xxxxxx@ccil.org
The first thing you learn in a lawin' family is that there ain't
no definite answers to anything.  --Calpurnia in To Kill A Mockingbird