SWISH -- SWI-Prolog for SHaring

%this has problems, interesting problems
:- op(500,yfx,>>).
:- op(500,xfy,<<).

:- discontiguous (>>)/2.
:- discontiguous (<<)/2.

%%A advanced depth first search that
%unifies To as it is found.
%
%by https://github.com/xpxaxsxi 
%License:
%Attribution 4.0 International (CC BY 4.0) 
%Attribution Required Starts Here
%Democracy!
%Save the Planet and stop most richest  capitalists!
%Mozilla Firefox!
%Swi-Prolog!
%https://github.com/xpxaxsxi 
%Attribution Required Ends Here
%
% (I don't believe this is a 
% novel algorithm ... and Swi-Prolog Chat GPT app
% confirmed this)
% route(From,To,[From])

t(A,B):-
    A>>B.
t(A,B):-
    B<<A.

route(From,To,Visited):-
    t(From,MidPoint),
    \+memberchk(MidPoint,Visited),
    VisitedNew=[MidPoint|Visited],
    (To=MidPoint;
    route(MidPoint,To,VisitedNew)).

1 >>  2. 2 >>  3. 3 >>  4. 4 >>  5.
         2          >>           5.
1            <<                  5.
         2  << 3. 3       <<     5.
         2        <<             5. 5     >> k.
                 
k >> m. m >> n.
k    <<      n.

a >> b. b >> c. c >> d.
a         <<         d.

/** <examples>
?- route(1,5,[1]).

What can be reached from 2 if 3 is blocked, can have duplicates
?- route(2,X,[2,3]).

What starting points are there to reach 2, can have duplicates
?- route(X,2,[X]).
*/