Problem Statement

It is known that the set of finite lists of positive integers is   countable.
This means that there exists a subset of natural integers which can be mapped to the set of finite lists of positive integers.

Implement such a mapping:

Demonstrate your solution by:

There are many ways to do this.   Feel free to choose any one you like.

Make the   rank   function as a   bijection   and show   unrank(n)   for   n   varying from   0   to   10.

Let's start with the solution: