BTW, I have found a strange behaviour of "modern" accidentals rule:
LilyPond considers volta alternatives as "previous" measure. Is this
correct? (Note the natural sign at the 2nd and 3rd alternatives, not
related to the previous _played_ measure.)
I do not know. I am not an expert on the different accidental rules,
nor on the standard engraving practices for accidentals in
alternatives. But I would expect that the last measure of the repeat
should serve as the previous measure for each of the alternatives.
And it should serve as the previous measure for starting ties and
slurs
and key and meter changes. And the first measure of the repeat should
serve as the following measure for unfinished ties and slurs for all
of
the alternatives except the last one (which means that tie and slur
directions should match and the tie/slur should extend to the bar line
rather than trigger an error).