A Prolog solver for the bridge and torch puzzle

bridge.pl

% A Prolog solver for the bridge and torch puzzle
% https://en.wikipedia.org/wiki/Bridge_and_torch_problem

print_all_solutions :-
    findall(_,print_solution,_).

print_solution :-
    init(State),
    solve(State,Solution,EndState),
    writeln('Start state:'),
    writeln(State),
    writeln('Solution:'),
    writeln(Solution),
    writeln('Final state:'),
    writeln(EndState), nl.

solve(State,[],State) :- goal(State).
solve(State,[Move|Tail],EndState) :- s(State,Move,NewState), solve(NewState,Tail,EndState).

goal(state([],right,[_,_,_,_],T)) :- T =< 17.

init(state([a,b,c,d],left,[],0)).

s(state(L1,left,L2,T),cross(L3),state(L4,right,L6,T2)) :-       
    select_one_or_two(L1,L3,L4),
    ord_union(L2,L3,L6),
    min_time_needed(L3,Tn),
    T2 is T + Tn, 
    T2 =< 17.
s(state(L1,right,L2,T),cross(L3),state(L4,left,L5,T2)) :-       
    select_one_or_two(L2,L3,L5),
    ord_union(L1,L3,L4),
    min_time_needed(L3,Tn),
    T2 is T + Tn, 
    T2 =< 17.

select_one_or_two(L,[Sel],L2) :- select(Sel,L,L2).
select_one_or_two(L,[Sel1,Sel2],L2) :- select(Sel1,L,NewL), select(Sel2,NewL,L2), Sel1@<Sel2.

min_time_needed([A],T) :- time_needed(A,T).
min_time_needed([A,B],T) :- time_needed(A,T1), time_needed(B,T2), max_list([T1,T2],T).

time_needed(a,1).
time_needed(b,2).
time_needed(c,5).
time_needed(d,10).