[boost] [proto] docs: Calc2 improvement proposal

Hi! First of all: This is a very nice example. Fantastic idea to introduce index information stored in the terminals at this place in the docs - perfect. If you define arg as template<int I> struct arg {}; and change the code accordingly the reader is not in need to know anything about mpl::int_ and IMHO the code template <int I> double operator ()(proto::tag::terminal, arg<I>) const { return d[ I - 1 ]; } is easier to read than template<typename I> double operator ()(proto::tag::terminal, arg<I>) const { return d[ I() - 1 ]; } Note that this should be written for beginners like me, so one catches the idea immediately without taking the extra path to mpl here. Markus