1 

     2 import C2._

     3 

     4 val urban_list1 = List("a", "b", "b", "c", "d")

     5 val urban_list2 = List("d", "b", "d", "b", "d")

     6 

     7 assert(prod(urban_list1, urban_list2) == 7)

     8 

     9 assert(prod(urban_list1, urban_list1) == 7)

    10 

    11 assert(prod(urban_list2, urban_list2) == 13)