Base list: 1 2 3 4 [ ] 6 7 8 9
Front list: 1 2 3 4
Back list: 9 8 7 6
User is expected to keep track of the "current element" and properly
fill it back in as necessary. (ToDo: Maybe it's more user friendly
to implicitly track the current element?)
Nota bene: the current class gets confused if you try to store NULLs
in the list.