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