⌂ Printing only odd bar numbers

• to a mode showing all bar numbers
• based on a calculation of the bar numbers to be visible.

Moreover, the Score.BarNumber #'break-visibility should be overridden e.g. to #all-visible

This is best placed in a \layout block to keep the music part "clean".

The first bar number can also be printed by inserting \bar "" before the music.

This modulo function can also be used in a more general way!