### 2.3.2 Music function usage

Music functions may currently be used in several places. Depending on where they are used, restrictions apply in order to be able to parse them unambiguously. The result a music function returns must be compatible with the context in which it is called.

- At top level in a music expression. No restriction apply here.
- As a post-event, explicitly started with a direction indicator (one of `-`, `^`, and `_`).

  In this case, you can’t use an open music expression as the last argument, one that would end with a music expression able to accept additional postevents.

- As a chord constituent. The returned expression must be of `rhythmic-event` type, most likely a `NoteEvent`.

The special rules for trailing arguments make it possible to write polymorphic functions like `\tweak` that can be applied to different constructs.

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#### LilyPond — Extending v2.18.2 (稳定分支).