% A model of movement - state is set of facts object at place
:-op(690, xfx, at).
:-dynamic(at/2).
is_place(X):-_ at X.
is_object(X):-X at _.

% 'put' is already in reserved for char output.

place(X,At):-assert(X at At), write(X), write(' is now at '), write_ln(At),!.

take(X,From):-retract(X at From),
        write(X), write(' was at '), write_ln(From),!.

move(X, To):-move(X,_,To).
move(X, From, To):-retract(X at From), 
	write(X), write(' was at '), write_ln(From),
	assert(X at To),
	write(X), write(' is now at '), write_ln(To),!.
d:-X at Y, write(X), write(' is at '), write_ln(Y), nl,
	fail.
q:-halt.