===== Unordered pair =====
==== Set ====
| @#55CCEE: context     | @#55CCEE: $  X,Y$ ... set |
| @#FFBB00: definiendum | @#FFBB00: $ x\in \{X,Y\} $ |
| @#55EE55: postulate   | @#55EE55: $ x = X \lor x = Y $ |

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$\{X,Y\} \equiv \{x \mid x = X \lor x = Y\}$

=== Discussion ===
$\{X,X\} = \{x \mid x = X \lor x = X\} = \{x \mid x = X\} = \{X\}$

=== Reference ===
Wikipedia: [[http://en.wikipedia.org/wiki/Axiom_of_pairing|Axiom of pairing]]

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=== Context ===
[[Sets]]