[concept.relation]

# 18 Concepts library [[concepts]](./#concepts)

## 18.7 Callable concepts [[concepts.callable]](concepts.callable#concept.relation)

### 18.7.5 Concept relation [concept.relation]

[ð](#concept:relation)

`template<class R, class T, class U>
  concept [relation](#concept:relation "18.7.5 Concept relation [concept.relation]") =
    [predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, T, T> && [predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, U, U> &&
    [predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, T, U> && [predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, U, T>;
`
