:- use_module(library(lists)).

plan(State,Goal,Plan) :-
    plan(State,Goal, [], Plan).

plan(State, Goal, Sol , Plan) :-
    subset(Goal, State),
    reverse(Sol, Plan).

plan(State, Goal, Current, Plan) :-
    action(A, Pre, Del, Add),
    subset(Pre, State),
    maplist(dif(A), Current),
    subtract(State, Del, State1),
    append(Add, State1, NewState),
    plan(NewState, Goal, [A | Current], Plan).

    
% action(action, preconditions, delete list, add list)
action(drive(X,Y), [at(X)], [at(X)], [at(Y), vis(Y)]) :-
    rel(X,Y).

con(perth, adelaide).
con(darwin, adelaide).
con(adelaide, sidney).
con(sidney, brisbane).

% symmetric closure
rel(X,Y) :- con(X,Y).
rel(Y,X) :- con(X,Y).

solve(P) :-
    plan([at(sidney), vis(sidney)],
         [at(sidney), vis(sidney),
          vis(adelaide), vis(brisbane), vis(perth), vis(darwin)],
         P).