10.11 Engraver tutorial
Engravers are C++ classes that catch music events and
create the appropriate grobs for display on the page. Though the
majority of engravers are responsible for the creation of a single grob,
in some cases (e.g. New_fingering_engraver), several different grobs
may be created.
Engravers listen for events and acknowledge grobs. Events are passed to
the engraver in time-step order during the iteration phase. Grobs are
made available to the engraver when they are created by other engravers
during the iteration phase.