Fib.java

TailRec<BigInteger> fib(final BigInteger n) {
  return n.compareTo(two) < 0
    ? TailRec.done(n)
    : Optional.ofNullable(memo.get(n)).map(TailRec::done).orElseGet(() ->
        TailRec.call(() -> fib(n.subtract(one))).flatMap(a -> 
          TailRec.call(() -> fib(n.subtract(two)).map(b -> {
            final BigInteger m2 = a.add(b);
            memo.put(n, m2);
            return m2;
          }))
        )
      );
}