-ADD_INTERFACE (Staff_symbol_referencer, "staff-symbol-referencer-interface",
- "An object whose Y position is meant relative to a staff "
- "symbol. "
- "These usually have @code{Staff_symbol_referencer::callback} "
- "in their @code{Y-offset-callbacks}. ",
+bool
+position_less (Grob *const &a, Grob *const &b)
+{
+ return Staff_symbol_referencer::get_position (a)
+ < Staff_symbol_referencer::get_position (b);
+}
+
+ADD_INTERFACE (Staff_symbol_referencer,
+ "An object whose Y@tie{}position is meant relative to a staff"
+ " symbol. These usually"
+ " have @code{Staff_symbol_referencer::callback} in their"
+ " @code{Y-offset-callbacks}.",