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'debug' is a common, and common-sense, variable name that can conflict with
a 'debug' parameter or variable in the stylesheet under test (as happened
to me). Perhaps this $debug should be $x:debug to avoid conflicts?
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(Anybody know a better way to make a link to particular lines in a
particular revision of a file that's in SVN?)
The variable isn't used anyway because you currently can't use variables in
@use-when attributes.