* lily/grob.cc (programming_error): add Grob::programming_error with
origin location.
* lily/tuplet-bracket.cc (parallel_beam): robustness check, don't
fail if a beam doesn't have stems.
* lily/engraver-group-engraver.cc (do_announces): scary change in
calling convention of create_grobs(): no create_grobs() call
before acknowledge_grobs().
* lily/sequential-music-iterator.cc (skip): add support for grace
notes.