FES
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In FES, there is a 
added before f
2 on the 3rd beat of the bar. The mark is of a cautionary nature, facilitating an a prima vista performance of this fragment. Marks of a similar function are present also in bars 91, 92 and 98.
Compare the passage in the sources »
category imprint: Differences between sources
issues: Annotations in teaching copies, Cautionary accidentals, Annotations in FES
notation: Pitch