code: def identifier_set(type, value)
        isbn_id = identifier(type)
  
        # create a new isbn record if we need to
        if isbn_id.nil?
          isbn_id = ONIX::ProductIdentifier.new
          isbn_id.product_id_type = type
          product.product_identifiers << isbn_id
        end
  
        isbn_id.id_value = value
      end
docs: set the value of a particular ID