ChordSymbols: addb7 gives doubly-flatted seventh

[Dazzid]

music21 version

9.8.7

Problem summary

music21 is encoding several chords wrongly. 1) Csus add b7, it seems no allowed this very common jazz chord, as it decoded it as ['C2', 'F2', 'G2', 'A3'] 2) Slash chords with extensions need to be corrected. For example, I'm parsing an XML file of a song with an F# b9 / B, and music21 decodes it as F#7/B add b9, which is a huge error. Is there a way to fix it? 

Steps to reproduce

element = 'Csus add b7'
tmp = m21.harmony.ChordSymbol(element)
theNotes = [str(p) for p in tmp.pitches]
print(theNotes)

Expected vs. actual behavior

More information

[jacobtylerwalls]

Csus add b7, it seems no allowed this very common jazz chord, as it decoded it as ['C2', 'F2', 'G2', 'A3']

Csusadd7 (or even "C7sus") gives what you want.

Looks like the redundant "b" in "b7" (perhaps a "Nashville number"?) is causing music21 to doubly-flat the 7th. We can avoid that, I'll open a PR.

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Slash chords with extensions need to be corrected. For example, I'm parsing an XML file of a song with an F# b9 / B, and music21 decodes it as F#7/B add b9, which is a huge error. Is there a way to fix it?

I'll need more information to reproduce. I created a file with the chord symbol "F# b9 / B", exported a musicxml file, imported that, and retrieved the expected pitches:

      <harmony print-frame="no">
        <root>
          <root-step>F</root-step>
          <root-alter>1</root-alter>
          </root>
        <kind>major</kind>
        <degree>
          <degree-value>9</degree-value>
          <degree-alter>-1</degree-alter>
          <degree-type>add</degree-type>
          </degree>
        <bass>
          <bass-step>B</bass-step>
          </bass>
        </harmony>

>>> s["ChordSymbol"].first().pitches
(<music21.pitch.Pitch B1>, <music21.pitch.Pitch F#2>, <music21.pitch.Pitch A#2>, <music21.pitch.Pitch C#3>, <music21.pitch.Pitch G3>)