===== Countable set =====
==== Collection ====
| @#FFBB00: definiendum | @#FFBB00: $X\in$ it |
| @#FFFDDD: exists      | @#FFFDDD: $f:\mathbb N\to X$ |
| @#55EE55: postulate   | @#55EE55: $f$ ... surjective |

==== Discussion ====
==== Parents ====
=== Requirements ===
[[Surjective function]], [[Natural number]]