Thanks to some terrific contributions and suggestions, especially by
new co-author Greg Snow, this version has a full GUI interface (native
on Windows, or using "tkrplot"), a puzzle generator, and a function to
fetch recently published puzzles. The GUI (playSudoku) can act as an
editor to input a puzzle, then give you hints, correct errors, reveal
cells, or show the full solution.
See README for some work remaining to be done, including a uniqueness
test, "notes" in the GUI, and a scoring mechanism.