function isEmpty()
    {
        // If $this->subfields is null, we must have deleted it
        if (!$this->subfields) {
            return true;
        }

        // Iterate through the subfields looking for some data
        foreach ($this->subfields as $subfield) {
            // Check if subfield has data
            if (!$subfield->isEmpty()) {
                return false;
            }
        }
        // It is empty
        return true;
    }