For SCHEME hackers and interested parties only

[REM at MIT-MC (Robert Elton Maas)]

JONL suggested I pass this idea along.
There are two ways to conditionally build a list (and return it).
(1) MAPCAN   (2) DO loop with conditional PUSH, with NREVERSE at return.
We believe that MAPCAN is cleaner, because SCHEME et al could more easily
co-routine it and thus avoid building structure if the only use of the list
(by the calling procedure) will be to map down it once and throw it away.
For example, LIBDOC;ARYFIL > is done the "wrong" way.