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<pre>
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Rule 4
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If X c U is fixed
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and u C U is varied,
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then the following are equivalent:
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R4a. u C X.
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R4b. [u C X].
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R4c. [u C X](u).
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R4d. {X}(u).
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R4e. {X}(u) = 1.
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</pre>
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