code: func (iter *Iterator) ReadBigInt() (ret *big.Int) {
  	str := iter.readNumberAsString()
  	if iter.Error != nil && iter.Error != io.EOF {
  		return nil
  	}
  	ret = big.NewInt(0)
  	var success bool
  	ret, success = ret.SetString(str, 10)
  	if !success {
  		iter.ReportError("ReadBigInt", "invalid big int")
  		return nil
  	}
  	return ret
  }
docs: // ReadBigInt read big.Int