(adsbygoogle = window.adsbygoogle || []).push({}); How do I explicate "A is countable"?

My attempt:

In set theory, every thing is a set, even functions. Thus when we say "A is countable in M" we mean that there is another set B in M that contains {naturals} and A as ordered pairs.

I'm having trouble spelling out the "as ordered pairs" part.

Is is: B in M that contains N and A in ordered pairs such that (a, n) where a ∈ A and n ∈ N and for every a there is exactly one corresponding n and for every n there is exactly one corresponding a?

Is there an easier way to write this?

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# How do I explicate A is countable ?

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