private static State<IDictionary<BigInteger, BigInteger>, BigInteger> FibM(BigInteger z)
{
return z <= 1
? z.Insert<IDictionary<BigInteger, BigInteger>, BigInteger>()
: from u in State.Get<IDictionary<BigInteger, BigInteger>, IMaybe<BigInteger>>(d => d.Get(z))
from v in u.Select(n => n.Insert<IDictionary<BigInteger, BigInteger>, BigInteger>()).GetOrElse(() =>
from r in FibM(z - 1)
from s in FibM(z - 2)
let t = r + s
from _ in State.Mod<IDictionary<BigInteger, BigInteger>>(d => d.Put(Tuple.Create(z, t)))
select t)
select v;
}