docs: def extract(self, disk, files, path='.'):
          """Extracts the given files from the given disk.
  
          Disk must be an integer (1 or 2) indicating from which of the two disks
          to extract.
  
          Files must be a list of dictionaries containing
          the keys 'path' and 'sha1'.
  
          Files will be extracted in path and will be named with their sha1.
  
          Returns a dictionary.
  
              {'extracted_files': [<sha1>, <sha1>],
               'extraction_errors': [<sha1>, <sha1>]}
  
          """
          self.logger.debug("Extracting files.")
          extracted_files, failed = self._extract_files(disk, files, path)
  
          return {'extracted_files': [f for f in extracted_files.keys()],
                  'extraction_errors': [f for f in failed.keys()]}