I used IronScheme, which allows both.

It seems clear that SRFI 60, the source of this operation, is fine with negative counts for rotating downward, though the documentation does not say so.  The best approach seems to be to check for a negative count before calling the R6RS routine and add (- end start) to it.  Alternatively, just copy the definition in srfi-143-impl.scm into srfi-143.sls.

-- 

John Cowan          http://vrici.lojban.org/~cowan        xxxxxx@ccil.org

One Word to write them all / One Access to find them,

One Excel to count them all / And thus to Windows bind them.

                --Mike Champion