The Empty Set, the Unordered Pair and the Singleton

¶ The empty set $\emptyset$ is the unique set with no element, given by Elements Are Sets, and the Empty Set and the Pair of Two Sets Exist and Are Unique §empty.

¶ For sets $x$ and $y$, the unordered pair $\{x,y\}$ is the unique set whose elements are exactly $x$ and $y$, given by Elements Are Sets, and the Empty Set and the Pair of Two Sets Exist and Are Unique §pair.

¶ For a set $x$, the singleton of $x$ is $\{x\}=\{x,x\}$; its only element is $x$, by Elements Are Sets, and the Empty Set and the Pair of Two Sets Exist and Are Unique §pair.

All three are defined set symbols.