The ordered pair associated to $A$ and $B$ is the set $\webleft (A,B\webright )$ defined by
\[ \webleft (A,B\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ \webleft\{ A\webright\} ,\webleft\{ A,B\webright\} \webright\} . \]
