LilyPond Contributor's Guide: 10.9.3 Write convert-ly rule

### 10.9.3 Write convert-ly rule

If the modification changes the input syntax, a convert-ly rule should be written to automatically update input files from older versions.

convert-ly rules are found in python/convertrules.py

If possible, the convert-ly rule should allow automatic updating of the file. In some cases, this will not be possible, so the rule will simply point out to the user that the feature needs manual correction.

#### LilyPond — Contributor’s Guide v2.24.4 (stable-branch).