[CM] s7: How to use pretty-print in write.scm

[bil at ccrma.Stanford.EDU]

I add to write.scm as I encounter unhandled cases -- I'll
add the one you mention.  Guile appears to use a version
of Marc Feeley's genwrite.scm (also in slib), with
new code added (in Guile 2) for this kind of case
(I'm just guessing from a glance at the diffs).