Tab completion

DKF: One thing that much software offers these days (e.g. bash, the NT cmd shell) is interactive tab completion. That is, you type part of a word, press the Tab key, and the software fills in the rest of the word for you. You can do this with Tcl/Tk, and this is much easier in Tcl 8.6 with the addition of the tcl::prefix command.

[TODO: put in an actual example]