Conditional include

[Lassi Kortela]

> Lassi: The fact that different Schemes have different conventions for 
> where their library files are is actually a great convenience to me when 
> developing SRFIs.  All the actual code goes into files named *-impl.scm 
> (or multiple names if there are multiple files).  Then each library file 
> or equivalent, which is where the differences are concentrated, is in an 
> implementation-specific place.  The library file for (foo bar) will be at:
>
> Chibi:  foo/bar.sld
> Chicken: foo.bar.scm
> Guile: foo/bar.scm
> Ypsilon: foo/bar.sls, or foo/bar.ypsilon.sls if it is Ypsilon-specific
>
> And since each of these needs to be slightly different, that's a Good Thing.

This is suboptimal: the meaning is obscured, and things work by accident 
since you have only 4 Schemes and all of them happen to use different 
naming conventions. If you add more Schemes, some of them will 
eventually clash.

R6RS implementations have a convention of appending the implementation's 
name to the filename. For example, 
<https://github.com/arcfide/chez-srfi/tree/master/%253a39> has the files:

parameters.sls  ; Portable version of the code.
parameters.chezscheme.sls
parameters.ikarus.sls
parameters.ironscheme.sls
parameters.mzscheme.sls
parameters.ypsilon.sls

This is clearer and more likely to avoid clashes.

Best of all is the R7RS solution:

(define-library (foo)
  (cond-expand (chicken (include "foo.chicken.scm"))))