Dial User Interface on Navigation System

I was playing with (or watch someone else playing with) a onboard navigation system in a Mercedes in Germany. I think it is a after market unit, may not be made by Mercedes. Anyway it uses a dial to input addresses, i.e. typing. It would seems tedious at first, "dialing" through the alphabet. However the system is smart enough to only show the valid set of inputs at each point. i.e if you are spelling a street address, it knows at any point what are the valid names, and only display the letters that are valid. (This is similar to the search input in the Palm Desktop), and will even prefill with letters when those are the only choice.