question: What color is the object that is to the left of the purple sphere and behind the yellow sphere?
#1 return the   purple sphere  
#2 return the   yellow sphere  
#3 return objects 
#4 return #3 that  are to  the   left of #1 
#5 return #3 that  are behind #2 
#6 return #3 in  both  #4 and #5 
#7 return the   color of #6