The pairing of $X$ and $Y$ is the set $\webleft\{ X,Y\webright\} $ defined by

\[ \webleft\{ X,Y\webright\} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ x\in A\ \middle |\ \text{$x=X$ or $x=Y$}\webright\} , \]

where $A$ is the set in the axiom of pairing, of .


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