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

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

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

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

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