written in by mistake or by a foreign hand in 
in The natural visible in A (→GE1) before the upper note of the last sixth of the triplet is erroneous, which can be proven by the analogous bar 5, as well as Chopin's proofing of FE and the correction in GET. In its visual form, that natural does not much differ from those put by Chopin, so we cannot be certain that it was written by someone else. However, if we take into account the additions in bars 51-52, the interference is probable also here.
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category imprint: Interpretations within context; Differences between sources
issues: Annotations in teaching copies, Foreign hand additions in manuscripts, Authentic corrections of FE, Annotations in GET
notation: Pitch