Allow square brackets to indicate a clef was supplied

[BaMikusi]

The [square bracket] option should arguably also be available for clefs, especially if entering one will be required, even if in the source there is none.

[BaMikusi]

To be considered in particular: if the clef is missing, and there is not much context (e.g. sketches), the PAE interpretation may become absolutely hypothetical (since the cataloger must make up his/her mind about the presumably right clef and enter the pitches accordingly). This is such a substantial intrusion on the part of the cataloger that the more visible it becomes to the user, the better -- so, rather than merely say in the note field that "the clef has been supplied," better make the addition strike the eye in the rendering of the incipit itself.

[ahankinson]

I am actually in favour of this, since it means that we can really require something in the clef field, and not continually fight battles about "diplomatic" transcription.