path(X) :- path(X, []).
path([], _).
path([_], _).
path([A, B | T], VISTED) :-
	passage(A, B),
	not(member(B, [A | VISTED])),
	path([B | T], [A | VISTED]).
	
first(H, [H | _]). 

last(L, [L]).
last(L, [_ | T]) :- last(L, T).

tail(T, [_ | T]).

safe([]).
safe([_]).
safe([H | T]) :- not(grue(H)), safe(T).

safe_path(X) :-
	path(X),
	safe(X).


% Possibility one. Safe path from start to finish.
navigate(X) :-
	safe_path(X),
	first(F, X), start(F),
	last(L, X), end(L).

% Possibility two. Safe path from start to sword. Path from sword to finish.
navigate(X) :-
	append(START_TO_SWORD, TO_FINISH, X),
	tail(TO_FINISH, SWORD_TO_FINISH),
	sword(SWORD_POS),
	end(FINISH_POS),
	start(START_POS),
	safe_path(START_TO_SWORD),
	first(START_POS, START_TO_SWORD),
	last(SWORD_POS, START_TO_SWORD),
	path(SWORD_TO_FINISH),
	first(SWORD_POS, SWORD_TO_FINISH),
	last(FINISH_POS, SWORD_TO_FINISH).