The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
class Nil { };
template<typename HeadT, typename TailT = Nil>
class Cons {
public:
using Head = HeadT;
using Tail = TailT;
};