cluttered(Y) :- on(X,Y)
clear(X) :- block(X) & ~cluttered(X)

supported(X) :- on(X,Y)
table(X) :- block(X) & ~supported(X)

stack(X,Y,Z) :- on(X,Y) & on(Y,Z)

above(X,Z) :- on(X,Z)
above(X,Z) :- on(X,Y) & above(Y,Z)