lgtm
http://codereview.appspot.com/24044/diff/1/2 File input/new/fretboard-chordchanges.ly (right): http://codereview.appspot.com/24044/diff/1/2#newcode8 Line 8: \version "2.13.0" "2.13.1" http://codereview.appspot.com/24044/diff/1/3 File input/regression/fretboard-chordchanges.ly (right): http://codereview.appspot.com/24044/diff/1/3#newcode8 Line 8: \version "2.13.0" "2.13.1" http://codereview.appspot.com/24044/diff/1/5 File lily/fretboard-engraver.cc (right): http://codereview.appspot.com/24044/diff/1/5#newcode39 Line 39: SCM last_fret_notes_; scm_gc_mark () to prevent garbage collection? http://codereview.appspot.com/24044/diff/1/6 File ly/engraver-init.ly (right): http://codereview.appspot.com/24044/diff/1/6#newcode24 Line 24: \consists "Separating_line_group_engraver" Instrument_name_engraver?