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is a partition of A. Moreover, each partition P of A determines an equivalence relation
<math>\{(x,y) \mid (\exists A \in P.x \in A \wedge y \in A)\}</math>.
This technique has limitations in both [[ZFC]] and [[NFU]]. In [[ZFC]], since the universe
is not a set, it is seems possible to abstract features only from elements of small domains.
This can be circumvented using a trick due to [[Dana Scott]]: if R is an equivalence
relation on the universe, define <math>[x]_R</math> as the set of all y such that <math>y R x</math>
and the [[rank (set theory)|rank]] of y is less than or equal to the rank of any <math>z R x</math>. This works
because the ranks are sets.
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