concept MutableColorBaseConcept<ColorBaseConcept T> : Assignable<T>, Swappable<T> {
template <int K> struct kth_element_reference_type; where Metafunction<kth_element_reference_type>;
template <int K> kth_element_reference_type<kth_element_type<T,K>::type>::type at_c(T);
template <ColorBaseConcept T2> where { ColorBasesCompatibleConcept<T,T2> }
T& operator=(T&, const T2&);
};