pl 18:
	- robust Staff_commands

pl 17:
	- robust Input_commands
	- merged Vertical_/Horizontal music
	- PARTIALs with duration
	- modularized lexer: multiple runs

pl 16
	- notenames configurable

pl 15
	- grouping code
	- multibeams (simple)

pl 14
	- symtables from .ly
	- Input_command
	- Input_{score/staff}

pl 13
	- Horizontal/Vertical music
	- Input_commands
