code: def intersect_keys(keys, reffile, cache=False, clean_accs=False):
      index = None
      if cache:
          refcache = reffile + '.sqlite'
          if os.path.exists(refcache):
              if os.stat(refcache).st_mtime < os.stat(reffile).st_mtime:
                  logging.warn("Outdated cache; rebuilding index")
              else:
                  try:
                      index = (SeqIO.index_db(refcache,
                                              key_function=clean_accession)
                               if clean_accs
                               else SeqIO.index_db(refcache))
                  except Exception:
                      logging.warn("Skipping corrupted cache; rebuilding index")
                      index = None
      else:
          refcache = ':memory:'
      if index is None:
          index = (SeqIO.index_db(refcache, [reffile], 'fasta',
                                  key_function=clean_accession)
                   if clean_accs
                   else SeqIO.index_db(refcache, [reffile], 'fasta'))
      if clean_accs:
          keys = (clean_accession(k) for k in keys)
      for key in keys:
          try:
              record = index[key]
          except LookupError:
              logging.info("No match: %s", repr(key))
              continue
          yield record