%  Prolog Clauses for a Simple Blocks World

%  Action Precondition Axioms.

poss(move(X,Y),S) :- clear(X,S), clear(Y,S), not X = Y.
poss(moveToTable(X),S) :- clear(X,S), not ontable(X,S).

%  Successor State Axioms.

clear(X,do(A,S)) :- (A = move(Y,Z) ; A = moveToTable(Y)), on(Y,X,S) ;
                    clear(X,S), not A = move(Y,X).
on(X,Y,do(A,S)) :- A = move(X,Y) ;
                   on(X,Y,S), not A = moveToTable(X), not A = move(X,Z).
ontable(X,do(A,S)) :- A = moveToTable(X) ;
                      ontable(X,S), not A = move(X,Y).

%  Primitive Action Declarations.

primitive_action(move(X,Y)).    primitive_action(moveToTable(X)).