The Fibonacci sequence f(1), f(2), f(3),.. is:
		1, 1, 2, 5, 8, 13, 21,  34, 55......
	
		As you see the definition is easy to grasp:
					
		f(1) = f(2)= 1 
        f(n) = f(n-2) + f(n-1), if n >= 3
	
	
		Example:
		?- fib(6,R).
		R = 13
	
		fib(1,1).
		fib(2,1).
		fib(N,R):- N >= 3,N1 is N - 1,N2 is N - 2,
			   fib(N1,R1),fib(N2,R2),R is R1 + R2.