The MusicXML value of the accidental mark above, e.g. "sharp-sharp", which AccidentalAbove doesn't tell apart
from "double-sharp" (like Pitch.AccidentalXml for notes).
AccidentalBelowXml
AccidentalBelowXml:string
The MusicXML value of the accidental mark below, see AccidentalAboveXml.
The MusicXML value of the accidental mark above, e.g. "sharp-sharp", which AccidentalAbove doesn't tell apart from "double-sharp" (like Pitch.AccidentalXml for notes).