Now, let's make it our problem to realize a "sequence".
Can you have any idea?
"Ad hoc" strategy won't do.
You cannot design infinite number of elements so that any two are different.
And even if it is possible, we cannot memorize it, and accordingly, cannot use it as number.
In fact, you know the answer. You learned it in elementary school mathmatics. Let's recall it.
under a rule; each combination being an element of the sequence.
Thus an endless sequence can be realized.
This way of realization of a sequence is called "decimal generation."
And the natural number that is decimally generated is called the "decimal number."