New syntax for separating gabc and nabc voices

[davidweichiang]

Closes #1667.

• & advances to the next voice, but doesn't cycle back to the first voice.
• | is for cycling back to the first voice, but for backwards compatibility also can be used in place of &.

Currently, nabc-lines: is still required.

[davidweichiang]

This works as far as I know, but I just left it as a draft in case there was discussion about what the syntax should look like.

[rpspringuel]

What would removing the nabc-lines header requirement involve?

[davidweichiang]

My recollection is that the nabc voices are stored in an array, so the nabc-lines: is needed in order to malloc the array. Removing the nabc-lines: header would require adding code to realloc the array whenever a new voice is seen.

Tangentially, this is also related to a question that came up on the mailing list (https://groups.google.com/g/gregorio-users/c/IwslfmnTPDk); currently \gabcsnippet does not allow nabc because it doesn't offer a way to pass the nabc-lines: header.

[rpspringuel]

So which would be easier, in your opinion, removing the nabc-lines header or creating a way to pass headers to \gabcsnippet?

[davidweichiang]

I think they're both doable but the latter would be easier (and orthogonal to this PR).

I actually kind of like the nabc-lines header, since it's a global property of a score.

[davidweichiang]

This for me went on the back burner when @lbssousa said that "this proposal is not needed anymore." If it is still needed, then let's iron out the syntax. I'm looking at this PR and do not understand why I chose this syntax. I would think it would be better, and still backwards compatible, to make | mean "if at last voice, cycle back to first voice, else advance to next voice" and make & mean "if at last voice, cycle back to first voice, else error."

[lbssousa]

This for me went on the back burner when @lbssousa said that "this proposal is not needed anymore."

In fact, I've said "this proposal was not needed anymore" for syntax highlighting purposes (since a LSP server for Gregorio could be smart enough to identify the GABC/NABC alternations by semantics), but it would still be useful for other purposes (getting rid of nabc-lines header[*], better treatment of error cases like the one pointed in #1726, etc.).

I would think it would be better, and still backwards compatible, to make | mean "if at last voice, cycle back to first voice, else advance to next voice" and make & mean "if at last voice, cycle back to first voice, else error."

I agree (this syntax would resemble the one adopted in MusixTeX: | for intra-instrument staff alternating, and & for instrument alternating), and would suggest a deprecation warning about that "if at last voice, cycle back to first voice" for | delimiter, leading to a breaking change in Gregorio 7.0.0.

[*] One suggestion is implementing a "multi-stage parsing" of GABC files. In the first stage, the parser would calculate the maximum number of NABC voices within a snippet in the score (which would be easier to do with this new proposed syntax), and set nabc-lines to this value.