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is a relation of equality in $X$. If $\left(a\text{,}b\right)\in R$, then we prefer to write $a=b$. Another example is

$R=\left\{\left(a\text{,}A\right)\in X×{2}^{X}:a\in A\right\}\text{,}$

which defines a membership relation between elements and subsets of $X$. For $\left(a\text{,}A\right)\in R$, we write $a\in A$. One more example is

$R=\left\{\left(A\text{,}B\right)\in {2}^{X}×{2}^{X}:A\subseteq \right\}$

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