10.11.2 Translation process
At each timestep in the music, translation proceeds by calling the following methods in turn:
start_translation_timestep () is called before any user
information enters the translators, i.e., no property operations
(\set, \override, etc.) or events have been processed yet.
process_music () and process_acknowledged () are called
after all events in the current time step have been heard, or all
grobs in the current time step have been acknowledged. The latter
tends to be used exclusively with engravers which only acknowledge
grobs, whereas the former is the default method for main processing
within engravers.
stop_translation_timestep () is called after all user
information has been processed prior to beginning the translation for
the next timestep.