+;; The signature here is slightly fishy since the "fallback return
+;; value" is not actually music but #f. This used to be (void-music)
+;; but triggered "Parsed object should be dead" warnings for music
+;; objects outside of the current parser session/module. The called
+;; functions always deliver music and are used from the parser in a
+;; manner where only the last argument is provided from outside the
+;; parser, and its predicate "scheme?" is always true. So the
+;; fallback value will never get used and its improper type is no
+;; issue.