> You can use (library (scheme complex)) and (library (scheme inexact)) to
> determine the presence of complex and inexact numbers, which is why
> there are no features for them.

That's nice for R7RS code, but doesn't work for non-R7RS implementations
which can also make use of cond-expand.

Then again, not all R6RS implementations even have cond-expand out of
the box. Since R6RS has a library system largely compatible with R7RS,
it ought to be reasonably simple to support the (library ...) clause in
a R6RS cond-expand implementation.