question: show me the cheapest one way flight from san francisco to boston leaving san francisco after 9pm 
#1 return flights 
#2 return #1 from  san  francisco 
#3 return #2 that  are leaving san  francisco  after 9pm 
#4 return #3 to boston 
#5 return #4 that  are one way 
#6 return the cheapest #5