Algorithmic automation of the complex music notation rules in MEI–XML through the example of the accidentals: The article demonstrates how music notation rules can be formalized into a computer algorithm. In particular, it handles the rule, whether the accidentals should be rendered or not when they repeat on the notes of the same pitch in the same measure. The decision process is described step-by-step while referring to the music notation rules. The algorithm was implemented in the XSLT-language for the needs of the Digital Interactive Mozart Edition and can be applied to MEI data. The tool can be downloaded from <https://github.com/ismdme/DIME-tools>.