Run (w/ trace):
?- [planer].
?- trace.
?- state1(State), plan(State, [on(a,b)], Plan, FinState).
?- notrace.
state1 is:
% A state in the blocks world
%
% c
% a b
% ====
% place 1234
state1( [ clear(2), clear(4), clear(b), clear(c), on(a,1), on(b,3), on(c,a) ] ).