THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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You can run the command with multiple filenames, for example all the modified files that are about to be committed. (On a modern laptop, it only takes about 0.1 seconds to run xmllint --noout on all the XML files in the topics/ subdirectory, so you can be generous about validating frequently or for more files than strictly necessary.)
If everything is OK, the command produces no output. If there is any output, it will show the approximate line(s) where the XML error occurs.
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