identifiers

[Mike Amundsen]

While creating identifiers for fret diagrams I found the identifiers
can be alpha only. I've searched the archives to see if there was a
discussion that described the reasons for that constraint. I'd guess
it's for reasons of simplicity and/or something to do with the
interoperability of C and Scheme.

Thanks to all who have contributed to the development of lilypond. I'm
still a beginner but having lots of fun creating some great looking
output.

Mike