Reading global variables from Scheme [documentation request?]

[Jérôme Plût]

I would like to *read*, from within a Scheme function
(define-music-function), the values of variables such as
Score.currentBarNumber or Score.alternativeNumberingStyle.

I can have a guess that this is possible with such functions as
ly:context-property or ly:output-def-lookup, but the documentation of
these functions is quite terse.

How is it possible to access these values?

Thanks,

-- 
        Jérôme