A SelectList is a nonempty list which always has exactly one element selected.
It is an example of a list zipper.
myList =
SelectList.fromLists [ 1, 2 ] 3 [ 4, 5, 6 ]
SelectList.before myList
--> [ 1, 2 ]
SelectList.after myList
--> [ 3, 4, 5 ]
SelectList.selected myList
--> 3