code: func (it *HistoryIterator) Next() (string, *token.TokenTransaction, error) {
  	if it.current >= len(it.keys) {
  		return "", nil, io.EOF
  	}
  
  	txID := it.keys[it.current]
  	it.current++
  	tx, err := it.pool.TxByID(txID)
  
  	return txID, tx, err
  }
docs: // Next gets the next transaction from the history, or io.EOF if there are no more transactions.