Handling manual measure durations in musicxml2ly

[Andrew Dunning]

Dear list,

I am importing some scores of unmetered music into Lilypond via MusicXML, and 
naturally Lilypond is producing far superior results to Finale. To make the 
files appear as they do in other programs, I am trying to work out how to add 
support for manual measure durations in musicxml2ly.

This allows the composer to insert any number of beats into a particular 
measure regardless of the time signature, using this markup as a child of 
<measure>:

<backup>
    <duration>19</duration>
</backup>

Here is the MusicXML documentation, with an example:

https://usermanuals.musicxml.com/MusicXML/Content/EL-MusicXML-backup.htm

It appears to me that adding \cadenzaOn and \cadenzaOff before and after the 
relevant measures is the simplest approach, but I would appreciate knowing 
about other use cases that might suggest another approach:

http://lilypond.org/doc/v2.19/Documentation/notation/displaying-rhythms.html#unmetered-music

I have not yet determined the best way of modifying the code to achieve this, 
having only written basic Python scripts in the past, but am hoping to work it 
out. If anyone else is able to help, I am happy to provide any number of 
example files.

All best,

Andrew Dunning