def _referenced_object_ids
      @data.each_value.select { |v| v && v.respond_to?(:is_poxreference?) }.
        map { |o| o.id }
    end
def _referenced_object_ids
      @data.each_value.select { |v| v && v.respond_to?(:is_poxreference?) }.
        map { |o| o.id }
    end
Return a list of all object IDs of all persistend objects that this Hash
 is referencing.
 @return [Array of Integer] IDs of referenced objects