https://codereview.appspot.com/5569045/diff/3001/input/regression/lilypond-book/GNUmakefile File input/regression/lilypond-book/GNUmakefile (right):
https://codereview.appspot.com/5569045/diff/3001/input/regression/lilypond-book/GNUmakefile#newcode36 input/regression/lilypond-book/GNUmakefile:36: # Prevent parallel lilypond-book instances for this subdir I know this is a long time ago, but can you remember why this was added? https://codereview.appspot.com/5569045/
