Sentence:I had to battle through an [e1]aurora[/e1] of [e2]polar bears[/e2] to get to the station, where all the trains were cancelled and the one that I finally got took 37 hours to get to London, where there is no snow and it is about 20C like a summer day.
 e1: aurora. 
 e2 : polar bears.
Answer: Member-Collection(e2,e1)