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See comment 13 of issue 3056:
http://code.google.com/p/lilypond/issues/detail?id=3056. At the moment,
when constructing an event function for a text spanner, one can't rely
on using ^ or _ to set the direction of the spanner if one wants to
tweak bound-details.right.text according to the direction, as that
sub-property is not looked up - the direction has to be passed as an
additional parameter - see comment 7 as an example of doing that.