'this' due to the non-prefix 'rootfilesin'-kind matcher being mixed
          # in.
          'foo/bar' -> 'this'
          'qux' -> 'this'

        Important:
          Most matchers do not know if they're representing files or
          directories. They see ['path:dir/f'] and don't know whether 'f' is a
          file or a directory, so visitchildrenset('dir') for most matchers will
          return {'f'}, but if the matcher knows it's a file (like exactmatcher
          does), it may return 'this'. Do not rely on the return being a set
          indicating that there are no files in this dir to investigate (or
          equivalently that if there are files to investigate in 'dir' that it
          will always return 'this').
        ó