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) ] ).