% Figure 11.7   A depth-first search program that avoids cycling.


% solve( Node, Solution):
%    Solution is an acyclic path (in reverse order) between Node and a goal

solve( Node, Solution)  :-
  depthfirst( [], Node, Solution).

% depthfirst( Path, Node, Solution):
%   extending the path [Node | Path] to a goal gives Solution

depthfirst( Path, Node, [Node | Path] )  :-
   goal( Node).

depthfirst( Path, Node, Sol)  :-
  s( Node, Node1),
  not member( Node1, Path),                % Prevent a cycle
  depthfirst( [Node | Path], Node1, Sol).