BIA - L3

abs(X,Y) :- X<0, Y is -X.
abs(X,Y) :- X>=0, Y is X.


fibonacci(0,0) :- !.
fibonacci(1,1) :- !.
fibonacci(N,F) :-
	N_1 is N-1,
	fibonacci(N_1, F_1),
	N_2 is N-2,
	fibonacci(N_2, F_2),
	F is F_1+F_2.


divisor(X,Y,D) :-
	Y=0,
	D is X.
divisor(X,Y,D) :-
	Y=\=0,
	Y_dif is X mod Y,
	X_dif is Y,
	divisor(X_dif,Y_dif,D).


evaluating(X,X) :- number(X).
evaluating(X+Y,R) :-
	number(Y),
	evaluating(X,Y_1),
	plus(Y,Y_1,R).
evaluating(X-Y,R) :-
	number(Y),
	evaluating(X,Y_1),
        plus(Y,R,Y_1).