fibonacci algorism implementation of Nemerle with memoization

fibonacci.n

using Nemerle;
using System.Console;
using System.Numerics;

module Program
{
  Main() : void
  {
    def fib(n)
    {
      WriteLine("{0,8} : {1}", n, Fibonacci(n));
      when (n % 10 == 0) _ = ReadLine();
      fib(n + 1);
    }
    
    fib(1);
  }
  
  [Memoize]
  Fibonacci(n : int) : BigInteger
  {
    | _ when n <= 1 => 1
    | _ => Fibonacci(n - 1) + Fibonacci(n - 2)
  }
}

fibonacci_like_py.n

#pragma indent

using Nemerle
using System.Console
using System.Numerics

module Program
  Main() : void
    def fib(n)
      WriteLine("{0,8} : {1}", n, Fibonacci(n))
      when (n % 10 == 0) _ = ReadLine()
      fib(n + 1)
    
    fib(1)
  
  [Memoize] \
  Fibonacci(n : int) : BigInteger
    | _ when n <= 1 => 1
    | _ => Fibonacci(n - 1) + Fibonacci(n - 2)