code: func (f Uint64Field) DecInBatch(batch *leveldb.Batch) (val uint64, err error) {
  	val, err = f.Get()
  	if err != nil {
  		if err == leveldb.ErrNotFound {
  			val = 0
  		} else {
  			return 0, err
  		}
  	}
  	if val != 0 {
  		val--
  	}
  	f.PutInBatch(batch, val)
  	return val, nil
  }
docs: // DecInBatch decrements a uint64 value in the batch
  // by retreiving a value from the database, not the same batch.
  // This operation is not goroutine save.
  // The field is protected from overflow to a negative value.