printTrivialVoltaRepeats context property

Setting this option notates \repeat volta 1 with bar lines and, if there is an \alternative, with a bracket. The default remains not to notate it.

The implementation sends volta-span events and sets repeatCommands even for trivial repeats, but provides repeat/return counts so that engravers can choose to ignore them based on the value of the new property.

This option is set in InternalGregorianStaff so that a trivial repeat creates a bar line in a GregorianTranscriptionStaff with the default settings. The missing bar line was a shortcoming and is the main motivation for this feature.

Another potential use is for an ad-lib repeat that the user wants performed only once in the MIDI. The user would indicate the optionality out-of-band, say by using \alternative and overriding the bracket text.