finding a cycle path in prolog - prolog

Question

atom_elements(h1,hydrogen,[c1]).
atom_elements(n1,nitrogen,[o1, o2, c2]).
atom_elements(o1,oxygen,[n1]).
atom_elements(o2,oxygen,[n1]).
atom_elements(n2,nitrogen,[o3, o4, c4]).
atom_elements(o3,oxygen,[n2]).
atom_elements(o4,oxygen,[n2]).
atom_elements(h5,hydrogen,[c5]).
atom_elements(n3,nitrogen,[o5, o6, c6]).
atom_elements(o5,oxygen,[n3]).
atom_elements(o6,oxygen,[n3]).
atom_elements(h7,hydrogen,[c7]).
atom_elements(h8,hydrogen,[c7]).
atom_elements(h9,hydrogen,[c7]).
atom_elements(c1,carbon,[c2,c6,h1]).
atom_elements(c2,carbon,[c1,c3,n1]).
atom_elements(c3,carbon,[c2,c7,c4]).
atom_elements(c4,carbon,[c3,c5,n2]).
atom_elements(c5,carbon,[c4,c6,h5]).
atom_elements(c6,carbon,[c1,c5,n3]).
atom_elements(c7,carbon,[c3,h7,h8,h9]).
Example query on the 2,4,6-Trinitrotoluene:
?- tnt(X).
X = [[c1, [c2, n1, o1, o2], c3, [c4, n2, o3, o4], c5,
[c6, n3, o5, o6]]] .
My first step is to find a cycle path and show as a List. But the result is shown as fault. It cannot save the path as a list.
atom_elements(h1,hydrogen,[c1]).
atom_elements(n1,nitrogen,[o1, o2, c2]).
atom_elements(o1,oxygen,[n1]).
atom_elements(o2,oxygen,[n1]).
atom_elements(n2,nitrogen,[o3, o4, c4]).
atom_elements(o3,oxygen,[n2]).
atom_elements(o4,oxygen,[n2]).
atom_elements(h5,hydrogen,[c5]).
atom_elements(n3,nitrogen,[o5, o6, c6]).
atom_elements(o5,oxygen,[n3]).
atom_elements(o6,oxygen,[n3]).
atom_elements(h7,hydrogen,[c7]).
atom_elements(h8,hydrogen,[c7]).
atom_elements(h9,hydrogen,[c7]).
atom_elements(c1,carbon,[c2,c6,h1]).
atom_elements(c2,carbon,[c1,c3,n1]).
atom_elements(c3,carbon,[c2,c7,c4]).
atom_elements(c4,carbon,[c3,c5,n2]).
atom_elements(c5,carbon,[c4,c6,h5]).
atom_elements(c6,carbon,[c1,c5,n3]).
atom_elements(c7,carbon,[c3,h7,h8,h9]).
removeprevious(X,Y):-
X=Y.
filterList(A,In,Out) :-
exclude(removeprevious(A),In,Out).
nextlist(P,C,O) :-
atom_elements(C,_,L),
filterList(P,L,O).
findTNT(Start,Output):-
atom_elements(Start,_,List),
findTNT(Start,Start,List,[],Output).
findTNT(_,_,[],_,[]).
findTNT(Start,_,[H|_],_,Output) :-
H = Start,
write('Find it'), %For debugging%
append([H],[],Output).
findTNT(Start,Privious,[H|T],Visited,Output) :-
write(H+'--'), %For debugging%
H \== Start,
\+ member(H,Visited),
nextlist(Privious,H,List),
append([H],Visited,V),
(
findTNT(Start,H,List,V,Output),
Output = [],
findTNT(Start,Privious,T,V,Output);
append([H],Output,O),
Output is O).
The result is :

Answer 1

The error message is not very good, but it indicates that the problem is with your use of is. Concretely, line 65 of your code reads:
Output is O
At the problematic point in the program, Output is [] and O is [c6], which is why this goal is printed as
[] is [c6]
So the problem is that you are using is on something that is not an arithmetic expression. is is only usable for evaluating arithmetic, for example:
X is 2 + 2
It is not usable for equality of terms. Use = for that. So this last line should maybe be something more like:
Output = O
Except that that will also not work, because you are trying to "assign" a new term to a variable that is already bound to a different term. And you're trying to do "recursive append", which is something that is pretty much always wrong in Prolog. Some recent questions about this: Prolog - Recursively append numbers to a list, Prolog - Recursive append to list returning false