The singleton set containing $X$ is the set $\left\{ X\right\} $ defined by
\[ \left\{ X\right\} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ X,X\right\} , \]
where $\left\{ X,X\right\} $ is the pairing of $X$ with itself of Definition 4.3.3.1.1.
