% Players in a squash club are divided into three leagues, and players
% may only challenge members in their own league or the league below,
% if there is one.
% Write a Prolog program that will display all possible matches between
% club players in the form:
%    tom versus bill
% Use the cut to ensure for example, that
%    Tom versus bill
% and
%    bill versus tom
% are not both displayed.

% player_league(Player, League)

player_league(ann, 3).
player_league(joyce, 3).
player_league(ramona, 2).
player_league(cynthia, 2).
player_league(susan, 1).
player_league(lisa, 1).



possible_game(Player1, Player2) :-
		player_league(Player1, League1),
		player_league(Player2, League2),
		Player1 @< Player2,	% generate only canonical descriptions
		League1-League2 =< 1,	% difference not too great one way
		League2-League1 =< 1.	% difference not too great other way.

% write_all_games :-
% 		forall( possible_game(Player1, Player2)
% 		      , format('~w versus ~w.~n', [Player1,Player2])
%		      ).