> It's inherently not portable to do so, because the mapping from library
> names to files on the file system (whose existence can be checked) is
> not exposed by R6RS.  So you'd need one implementation of that mapping
> per functioning R6RS system

The Akku package manager currently has just such code to deal with the
special cases. The mapping to file names is not portable on R7RS either.
Even the .sld extension is not standard. I have a backward-compatible
solution in mind, it'd have to be prototyped to see if it'd be viable.