code: func (f *fragment) handleMutex(rowID, columnID uint64) error {
  	if existingRowID, found, err := f.mutexVector.Get(columnID); err != nil {
  		return errors.Wrap(err, "getting mutex vector data")
  	} else if found && existingRowID != rowID {
  		if _, err := f.unprotectedClearBit(existingRowID, columnID); err != nil {
  			return errors.Wrap(err, "clearing mutex value")
  		}
  	}
  	return nil
  }
docs: // handleMutex will clear an existing row and store the new row
  // in the vector.