-ADD_INTERFACE (Staff_symbol_referencer,"staff-symbol-referencer-interface",
- "Object whose Y position is meaning with reference to a staff "
-"symbol. Objects that have this interface should include "
-"Staff_symbol_referencer::callback in their Y-offset-callback. "
-,
- "staff-position");
+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 position is meant relative to a staff "
+ "symbol. "
+ "These usually have @code{Staff_symbol_referencer::callback} "
+ "in their @code{Y-offset-callbacks}. ",
+
+ /* properties */
+ "staff-position");
+
+