Find a software converter able to convert mscz, files to ly files.
In order to convert MuseScore backups (.mscz,) to LilyPond music scores (.ly), you would first need to recover the backup using MuseScore and then proceed to export the file to LY format. There does not appear to be alternative for this conversion and your only hope is the default software.
Unfortunately, there is no detailed software record in the database for this conversion.
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