Allows slurs to break at barlines.

lily/spanner.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1996--2012 Han-Wen Nienhuys <hanwen@xs4all.nl>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
 
@@ skipping 504 matching lines @@
 
 ADD_INTERFACE (Spanner,
                "Some objects are horizontally spanned between objects.  For"
                " example, slurs, beams, ties, etc.  These grobs form a subtype"
                " called @code{Spanner}.  All spanners have two span points"
                " (these must be @code{Item} objects), one on the left and one"
                " on the right.  The left bound is also the X@tie{}reference"
                " point of the spanner.",
 
                /* properties */
+               "other-half "
                "normalized-endpoints "
                "minimum-length "
                "spanner-broken "
                "spanner-id "
                "to-barline "
               );