abc2ly puts a version statement into its output files:
\version "2.5.20"
By design this trails the lilypond version, and is changed only when
abc2ly has been thoroughly tested with the current version.
However, when you run a file with an obsolete version through 2.7.39,
you get the following error messages:
Parsing...
error: Incorrect lilypond version: 2.5.20 (2.7.38, 2.7.39)
error: Consider updating the input with the convert-ly script
warning: identifier name is a keyword: `book'
score.ly: 0: warning: no \version statement found, add
\version "2.7.39"
for future compatibility
The first error is ok, although it might confuse someone about whether
they're running the correct version of abc2ly, but the one about "no
\version statement found" is both incorrect and confusing.