preferred extention for lilypond-book?

[Graham Percival]

What extension should I give to my lilypond-book files?  I can't find 
one that lilypond-book likes.

I normally use --output for my bigger lilypond-book projects, but what 
if I want to have a simple lilypond-book project that resides in a 
single directory?  It's not happy with .tex, .latex, .ly, or .lybook

If there isn't a preferred extention (something like .lybook would nice 
nice, IMO), then would it be a good idea to make one?

Still using fink 2.3.8

Cheers,
- Graham


spark-daemonology-net:~/tmp gperciva$ lilypond-book debug.tex
lilypond-book (GNU LilyPond) 2.3.8
lilypond-book: error: Output would overwrite input file; use --output.
spark-daemonology-net:~/tmp gperciva$ lilypond-book debug.latex
lilypond-book (GNU LilyPond) 2.3.8
lilypond-book: error: cannot determine format for: debug.latex
Traceback (most recent call last):
  File "/sw/bin/lilypond-book", line 1056, in ?
    main ()
  File "/sw/bin/lilypond-book", line 1053, in main
    do_file (files[0])
  File "/sw/bin/lilypond-book", line 905, in do_file
    output_filename = input_base + format2ext[format]
KeyError: 0
spark-daemonology-net:~/tmp gperciva$ lilypond-book debug.ly
lilypond-book (GNU LilyPond) 2.3.8
lilypond-book: error: cannot determine format for: debug.ly
Traceback (most recent call last):
  File "/sw/bin/lilypond-book", line 1056, in ?
    main ()
  File "/sw/bin/lilypond-book", line 1053, in main
    do_file (files[0])
  File "/sw/bin/lilypond-book", line 905, in do_file
    output_filename = input_base + format2ext[format]
KeyError: 0