Assignment

constexpr triplex<T>& operator=(const triplex<T>&) = default; 
triplex<T>& set(const T&, const T&, const T&);

These member functions assign the infinimum, value and supremum of the interval.

operator=(const triplex<T>&): Standard one-to-one assignment.
set(const T&, const T&, const T&): Set the infinimum $x$ , value $x$ and supremum $x$ of the triplex.