This is a CLIPS (http://clipsrules.sourceforge.net/) rule based system
that plays white king-rook against black king chess endgame.  To start
a game, after loading chess.clp, use (start-game).  Playing is pretty
self-explanatory.  The idea is not to do extensive search but use as
humane rules as possible.  Indeed, search goes as deep as 2 half-moves
at worst (mate situations) without even looking at all possible moves
and rules are pretty close to how an average player would reason in
such a simple endgame.  Understandably, not always does the system
find optimal move that leads to fastest mate.  However, most often it
finds some at least good enough move.  There is a parameter ?*N* which
determines size of board.  As far as tested, it always manages to
checkmate on boards bigger than 5x5.  However, it's been mainly tested
on 8x8.
Very few of the functions have side effects.  Realization has obvious
weaknesses like different rules computing same results in LHS and even
rules that invoke same functions in LHS and RHS.  However, at least on
8x8 board, performance is not a problem.