Why is `index_gen` in `multi_array` a `boost::deta

Question

This discussion has let to another question. It seems that index_gen in multi_array (a.k.a. boost::multi_array::index_gen) is set to boost::detail::multi_array::index_gen<0,0>, where I would have expected it to be boost::detail::multi_array::index_gen<Dim,Dim> where Dim is the template parameter of multi_array. Can anyone explain why this <0,0> type is used instead?