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How good a program? 

This is a serious question; if you can make the game more one of pattern
recognition than calculation, can you reduce the computer's ability to
'always' win? 

For example, Gary Gifford's Time Travel Chess [or whatever it's properly
named]; it allows players to go back and change previously-played moves -
how easy is this to program? 

The 'dark' chesses, those with imperfect information, they are more
difficult, and the computer would have to play those more like a wargame,
which has possibly perfect piece information, but semi-random combat
results. The computer has to use a more statistical approach to moves
there, and I would think it would also [have to] use a more statistical
approach in Kriegspiel, which is exactly opposite - totally determined
combat but unknown piece placement. So I'd suspect variants designed to
be more like wargames might reduce the computer's ability to crush its
human opponent. Others have disagreed.

What types of chess variants are hard to program?