Show the types of ships that have both ships with tonnage larger than 6000 and ships with tonnage smaller than 4000.
return ships   ;return tonnages of  #1 ;return #1 where #2 is  higher than 6000 ;return #1 where #2 is  lower than 4000 ;return #1 of  both #3 and  #4 ;return types of  #5
['select', 'project', 'comparative', 'comparative', 'intersection', 'project']