It would be fun if the SRFI collection could eventually be split into
zones based on how commonly a SRFI is implemented. This would indicate
how safe it is to use in portable code.

* Green zone - Available on all the major Scheme implementations.

* Yellow zone - Reasonably common, but not ubiquitous.

* Red zone - Rare.

Of course this would be just one way to map them, but perhaps the most
useful one for beginners (and why not advanced programmers as well).