= Combinatorial structure
A <combinatorial structure> is a discrete mathematical object specified by underlying elements and rules organizing them. A <graph> equips a <set> of <vertices> with <edges>; a <permutation> orders labels; a <set partition> <groups> them into nonempty <subsets>. <Counting> requires an explicit equality convention. Two <labelled combinatorial structures> may differ only by their labels, whereas an <unlabelled combinatorial structure> identifies objects related by an appropriate <isomorphism>. Thus there are three labelled two-element <subsets> of a three-element <set>, but only one abstract two-element <set> up to <bijection>.
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