code: def fetch(self, seq_id, start=None, end=None):
          
          rec = self._db.execute("""select * from seqinfo where seq_id = ? order by added desc""", [seq_id]).fetchone()
  
          if rec is None:
              raise KeyError(seq_id)
  
          if self._writing and self._writing["relpath"] == rec["relpath"]:
              logger.warning("""Fetching from file opened for writing;
              closing first ({})""".format(rec["relpath"]))
              self.commit()
  
          path = os.path.join(self._root_dir, rec["relpath"])
          fabgz = self._open_for_reading(path)
          return fabgz.fetch(seq_id, start, end)
docs: fetch sequence by seq_id, optionally with start, end bounds