blocks3.pl

% on(X,Y) means that block X is directly on top of block Y.
%on(b1,b2).   on(b3,b4).   on(b4,b5).   on(b5,b6).

%block(Name,X,Y)
block(b1,0,1).
block(b2,0,0).
block(b3,1,3).
block(b4,1,2).
block(b5,1,1).
block(b6,1,0).
block(b7,2,0).
% just left(X,Y) means that blocks X and Y are on the table
% and that X is immediately to the left of Y.
just_left(B1,B2):- 
    block(B1,X1,0), block(B2,X2,0),
    X1-X2 =:= -1.

% above(X,Y) means that block X is somewhere above block Y 
% in the pile where Y occurs.
% above(X,Y) :- on(X,Y).
% above(X,Y) :- on(X,Z), above(Z,Y).
above(B1,B2) :-
    block(B1,X,Y1), block(B2,X,Y2),
    Y1>Y2.


% % left(X,Y) means that block X is somewhere to the left 
% % of block Y but perhaps higher or lower than Y.
% % left(X,Y) :- just_left(X,Y).
% % left(X,Y) :- just_left(X,Z), left(Z,Y).
% % left(X,Y) :- on(X,Z), left(Z,Y).     % leftmost is on something.
% % left(X,Y) :- on(Y,Z), left(X,Z).     % rightmost is on something.
left(B1,B2) :-
    block(B1,X1,_), block(B2,X2,_),
    X1<X2.

% left(X,Y)  :-  bottom(X,X1),  bottom(Y,Y1),  table_left(X1,Y1). 

% bottom(X,X)  :-  \+  on(X,Y).	%  Note  the  use  of  \+  !
% bottom(X,X1)  :-  on(X,Y),  bottom(Y,X1).

% table_left(X,Z)  :-  just_left(X,Z).
% table_left(X,Z)  :-  just_left(X,Y),  table_left(Y,Z).


% % right(X,Y) is the opposite of left(X,Y).
% right(Y,X) :- left(X,Y).
right(B1,B2) :- 
    block(B1,X1,_), block(B2,X2,_),
    X1>X2.