list types of aircraft that connect boston and san francisco 
return flights ;return #1 from boston ;return #2 to  san francisco ;return #1 from san francisco ;return #4 to  boston ;return the  aircraft of #3 ;return the  aircraft of #5 ;return #6 ,  #7
['select', 'filter', 'filter', 'filter', 'filter', 'project', 'project', 'union']