1. Only show tune markers when a tune change is in the middle of a line.
2. Only show repeat starts |: if the repeat start is in the middle of a line.
3. Final bar end mark must be a thick line.
The official rules for a repeat are that if there is no left repeat
marker |: then the repeat starts at the previous double bar.
Digging around, I find that the tune is actually due to no less than
Mozart. Replace Nick Barber with what Mozart wrote. I'm not sure it's
still exactly what Havoc play, but I think it's closer.