+(ly:add-interface
+ 'trill-spanner-interface
+ "A trill spanner."
+ '())
+
+(ly:add-interface
+ 'trill-pitch-accidental-interface
+ "An accidental for trill pitch."
+ '())
+
+(ly:add-interface
+ 'unbreakable-spanner-interface
+ "A spanner that should not be broken across line breaks. Override
+with @code{breakable=##t}."
+ '(breakable))
+