list united flights from denver to san francisco on tuesdays or wednesdays 
return flights ;return #1 from  denver ;return #2 to san francisco ;return #3 that  are united ;return #4 on  tuesdays ;return #4 on  wednesdays ;return #5 , #6
['select', 'filter', 'filter', 'filter', 'filter', 'filter', 'union']