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A general technique for implementing abstractions in set theory is the use of equivalence
classes. If an equivalence relation R tells us that elements of its field A are alike in
some particular respect, then for any set x we can regard the set <math>[x]_R=\{y \in A \mid x R y\}</math> as
representing an abstraction from the set x respecting just those features (we identify
elements of A up to R).
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