xvier is a (not) simple game where you and the computer alternately
throw stones into free columns. The stones pile up in the columns,
and the goal is to get four stones in a row, column, or diagonally.
The default grid is 7 columns by 6 rows. It can be sized from 4x4 to
13x13. It has 10 levels of difficulty.
Provided by Jeremy C. Reed in PR#19100
