ly:book-scores includes other elements

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Dear LilyPond developers,

I am not sure if this is a bug or not, but judging by the documentation, 
the Scheme function `ly:book-scores` should only return scores. Instead, 
it includes other top-level elements like markup. In the attached 
example, I would have expected it to print only `#<Score>`, but I see 
the `(This is not a score!)` as well. Is this a desired behaviour?

Thanks in advance.

Jan "Singon" Slany

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