TRS:
 {           plus(0(), y) -> y,
            plus(s(x), y) -> s(plus(x, y)),
            lt(0(), s(y)) -> true(),
               lt(x, 0()) -> false(),
           lt(s(x), s(y)) -> lt(x, y),
                   fib(x) -> fibiter(x, 0(), 0(), s(0())),
      fibiter(b, c, x, y) -> if(lt(c, b), b, c, x, y),
  if(false(), b, c, x, y) -> x,
   if(true(), b, c, x, y) -> fibiter(b, s(c), y, plus(x, y))}
 Fail