Again a challenge from 99 Elm problems

Convert a list to a list of lists where repeated elements of the source list are packed into sublists. Elements that are not repeated should be placed in a one element sublist.

pack [1,1,1,2,3,3,3,4,4,4,4,5,6,6] ==
    [ [1,1,1]
    , [2]
    , [3, 3, 3]
    , [4, 4, 4, 4]
    , [5]
    , [6, 6]
    ]