The Neapolitan (major triad on b2 of a key that does not have that degree
already) is recognized first, since it sits outside both parallel modes;
parallel-mode borrowing follows the isBorrowedChord rules. Diatonic
chords and altered-minor chords, and unrecognized chromatic chords all yield
null.
Identify where a non-diatonic chord comes from.
The Neapolitan (major triad on b2 of a key that does not have that degree already) is recognized first, since it sits outside both parallel modes; parallel-mode borrowing follows the isBorrowedChord rules. Diatonic chords and altered-minor chords, and unrecognized chromatic chords all yield null.