[plt-scheme] Portable development

I'm a guy who's a bit fanatical about making sure that a program is 
"done right." What's getting me right now is the lack of good 
portability among implementations. My solution which gives me the 
closest "feeling of security" with portability is the following:

* Develop the core definitions in R5RS if possible, making any logic 
and back-end in the program completely useable by any R5RS 
compatible implementation. 

* Develop any front-ends in as much R5RS as possible, and 
modularize the others so that each front end can use the same 
back-end, and for each implementation, it is only necessary to 
change the superficial elments of the front-end.

* Divide the code into the parts which are implementation centric, 
and the portable parts, and then use portable access methods 
between them so that the implementation centric parts can easily 
be mixed and matched while keeping the same actual program 
intact.

Is this the right way to go about this? Is there a cleaner or more 
appropriate approach?

- Aaron Hsu <af at aaronhsu.com>