> Cool! I skimmed past cond-expand description and this (library ..)
> syntax went in one ear and straight out of the other.

It's a bit confusing with so many parentheses. But it doesn't check for
feature identifiers at all, only importable libraries. Example:

(cond-expand ((library (srfi 130)) (import (srfi 130)))
              ((library (srfi 13))  (import (srfi 13))))

(string-prefix? "a" "abc")

Since SRFI 130 is a superset of SRFI 13, that cond-expand will import
whichever one of those is available and you can then use the string
procedures with their familiar names. In Chibi it imports 130, in Gauche
and Kawa it imports 13.