:- use_module(lambda).
fib(N, FN) :-
cont_fib(N, _, FN, \_^Y^_^U^(U = Y)).
cont_fib(N, FN1, FN, Pred) :-
( N < 2 ->
call(Pred, 0, 1, FN1, FN)
;
N1 is N - 1,
P = \X^Y^Y^U^(U is X + Y),
cont_fib(N1, FNA, FNB, P),
call(Pred, FNA, FNB, FN1, FN)
).