i'd like a ticket on a plane from denver to atlanta i would like a coach class or lowest fare ticket 
return flights ;return #1 from  denver ;return #2 to atlanta ;return tickets of #3 ;return price of #4 ;return #4 where #5 is the  lowest ;return #4 that are coach class tickets ;return #7 ,  #6
['select', 'filter', 'filter', 'project', 'project', 'comparative', 'filter', 'union']