The singleton set containing $X$ is the set $\webleft\{ X\webright\} $ defined by
\[ \webleft\{ X\webright\} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ X,X\webright\} , \]
where $\webleft\{ X,X\webright\} $ is the pairing of $X$ with itself of Definition 2.3.3.1.1.
