What the heck is Y-offset-callback?

[Cameron Horsburgh]

Hi folks,

There's a snippet in the LSR which contains a reference to the
Y-offset-callbacks:

\override Score.VoltaBracket #'Y-offset-callbacks = #(list 
ly:staff-symbol-referencer::callback)

LilyPond compiles it, but warns me:

warning: can't find property type-check for Y-offset-callbacks'
(backend-type?).  perhaps a typing error?
warning: doing assignment anyway

I can't find any reference to the Y-offset-callbacks in the
VoltaBrackets section of the Program Reference. Can anyone tell me
what it is, if I need it and what I should do with it?
-- 

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Cameron Horsburgh

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