Support multiple accidentals

[infojunkie]

In the microtonal world, multiple accidentals are commonplace. I provide some examples here:

• Here's a score written in Sagittal notation. This system includes 2 modes of notation: A "pure" mode where most Just Intonation intervals can be described with a single symbol, and a "mixed" mode which combines a subset of the pure mode with conventional sharps and flats.

• Helmholtz-Ellis JI pitch notation (HEJI) is another notation system that also introduces accidentals that are meant to be combined, as per this excerpt from the HEJI guide:

• Easley Blackwood, "Modes and chord progressions in equal tunings". Perspectives of New Music vol. 29/2, 1991.

• Also noting that SMuFL expects those accidentals to be combined, c.f. implementation notes of Johnston accidentals.

What are the needed changes?

As per my previous discussion in this repo, I suggest to simply change the cardinality of node/accidental and key/key-accidental from 0-1 to 0-N. This will give microtonal engravers the flexibility they need and would impact neither existing scores nor software.

These 2 single-line modifications should be sufficient:

• eb96485#diff-d35ba23334135dd42cac4b98e63d68c077bcfd655747a9beaf26ac3bfedd213a

• e9b4d94#diff-d35ba23334135dd42cac4b98e63d68c077bcfd655747a9beaf26ac3bfedd213a

The documentation of accidentals should also set some recommendation as to the visual ordering of multiple accidentals, either following the order of declaration in the XML, or reversing it (which makes more sense to me since accidentals are notated before the actual note, so the ordering would proceed from the note rightward.)

[lemzwerg]

Looks reasonable to me. However, I would stay with visual order – AFAIK, there is nothing in the standard that encodes music notation stuff from 'right' to 'left'.

[infojunkie]

For what it's worth, MEI supports multiple accidentals and orders the accidentals from note rightward.

I submitted to Verovio the merged PR to ensure that multiple accidentals are correctly rendered in the context of chords, as per the guidelines in Behind Bars.

[davidmatthew-ie]

It would be great to get this functionality supported, and move MusicXML closer towards more fully accommodating non-Western and microtonal music notation.