1 

     2 

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

     4 

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