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<p>is sometimes called a <a href="page.php?w=setoid">setoid</a>, typically in <a href="page.php?w=type_theory">type theory</a> and <a href="page.php?w=proof_theory">proof theory</a>.</p>

<p><big> Definition and notation </big></p>
<p>A partition of a set X is a set of non-empty subsets of X such that every element x in X is in exactly one of these subsets (i.e., the subsets are nonempty mutually <a href="page.php?w=disjoint_sets">disjoint sets</a>).</p>

<p>Equivalently, a <a href="page.php?w=family_of_sets">family of sets</a> P is a partition of X if and</p><p>
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