===== Involution =====
==== Set ====
| @#55CCEE: context     | @#55CCEE: $X$ ... set |
| @#FFBB00: definiendum | @#FFBB00: $ f\in \mathrm{it}(X,X) $ |
| @#55EE55: postulate   | @#55EE55: $ f:X\to X $ |
| @#55EE55: postulate   | @#55EE55: $ f\circ f=\mathrm{id_X} $ |

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=== Subset of ===
[[Unary operation]]