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helloWorld :-
    write('HELLO WORLD').


factorial(0,1).
factorial(N,R)
 :- N1 is N-1,
   factorial(N1,R1),
   R is N * R1.
   
sum([],0).
sum([X|XS], R)
:- sum(XS,W),
    R is X + W.

maximum([X],X).
maximum([X|XS], X)
  :- maximum(XS, v),
        X > V.
maximum([X|XS], V)
:- maximum(XS,V),
    X =< V.

contains(X, [X|_]).
contains(X, [_|XS])
:- contains(X,XS).

reverse([],[]).
reverse([X|XS], W)
:- reverse(XS, V),
    append(V, [X], W).