2.3.7 Void music functions

A music function must return a music expression. If you want to execute a function only for its side effect, you should use define-void-function. But there may be cases where you sometimes want to produce a music expression, and sometimes not (like in the previous example). Returning a void music expression via #{ #} will achieve that.

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LilyPond — Extending v2.18.2 (stable-branch).