SWISH -- SWI-Prolog for SHaring

link(1,2).
link(2,3).
link(3,4).
link(3,6).
link(6,7).
link(6,5).

% ---------------------------------------------------------------------------
%
% traverse( Origin, Destination, ReversePath )
% 
% traverse/3 traverses a directed graph defined by link(From,To).
% The path taken is listed in reverse order:
% [Destination, N3, N2, N1 , Origin]
%
% ---------------------------------------------------------------------------
traverse(A,B,P) :-         % to traverse a graph
  traverse(A,B, [], P)     % - invoke the helper, seeding the visited list with the empty list
  .                        %

% ---------------------------------------------------------------------------
% traverse( Origin, Destination, Visited, ReversePath )
% ---------------------------------------------------------------------------
traverse( A , B , V , [B,A|V] ) :- % Traversal of a graph from A to B,
  link(A,B)                        % - succeeds if there exists an edge between A and B.
  .                                %
traverse( A , B , V , P     ) :-   % otherwise, we can get from A to B if...
  link(A,X) ,                      % - an edge exists between A and some node X
  \+ member(X,V) ,                 % - that we have not yet visited,
  traverse(X,B,[A|V],P)            % - and we can get from X to B (adding A to the list of visited nodes
  .                                %