code: def files(self):
          
          # maybe replace with a trie at some point? see has_file FIXME
          already_seen = set()
          filenames = []
  
          for el in self.xml.xpath("//class"):
              filename = el.attrib['filename']
              if filename in already_seen:
                  continue
              already_seen.add(filename)
              filenames.append(filename)
  
          return filenames
docs: Return the list of available files in the coverage report.