What is the id of the routes whose source and destination airports are in the United States?
return routes ;return source airports of  #1 ;return #1 where #2 is  in United States ;return destination airports of  #1 ;return #1 where #4 is  in United States ;return #1 of  both #3 and  #5 ;return ids of  #6
['select', 'project', 'comparative', 'project', 'comparative', 'intersection', 'project']