1 

     2 import CW7a._

     3 

     4 assert(occurrences(List("a", "b", "b", "c", "d")) == Map("a" -> 1, "b" -> 2, "c" -> 1, "d" -> 1))

     5 

     6 assert(occurrences(List("d", "b", "d", "b", "d")) == Map("d" -> 3, "b" -> 2))