David A. Wheeler scripsit:

> Is there a a simple example of an efficient R7RS "write" and
> "write-shared" implementation?  The "obvious" solution involves hash
> tables (which are not portable), & I fear there's some corner case or
> clever simplification I won't realize.

I don't know any other way to do it.  However, though not part of
R7RS-small, SRFI 69 hash tables are fairly pervasive, because they are
simple, have a reference implementation, and can be built on top of
R6RS hashtables (which are standard).  The only reason they aren't in
more Schemes is that as SRFIs go, 69 is a fairly recent one.  I wouldn't
hesitate to use them.

--
"But I am the real Strider, fortunately,"       John Cowan
he said, looking down at them with his face     xxxxxx@ccil.org
softened by a sudden smile.  "I am Aragorn son  http://www.ccil.org/~cowan
of Arathorn, and if by life or death I can
save you, I will."  --LotR Book I Chapter 10