Recovering the Solution Path

The new node data structure has a slot for the node's parent. We must make sure that it is correctly initialised when the node is created. Currently, this happens in two places.

• When the initial node is created.

  % start node
  node_initial(N) :-
   start_state(S),
   node_new(N),
   node_state_of(N,S).

• Whenever a node is expanded

  sl2nl([S|Y],[N|NL]) :-
   node_new(N),
   node_state_of(N,S),
   sl2nl(Y,NL).