Singleton Equality

Theorems
Let $x$ and $y$ be sets.

Then:


 * $\left\{{x}\right\} \subseteq \left\{{y}\right\} \iff x = y$
 * $\left\{{x}\right\} = \left\{{y}\right\} \iff x = y$

Proof
Then:

Source

 * : $\S 7.7$