inline typename boost::enable_if_c<
boost::unordered::detail::has_max_size<Alloc>::value, SizeType
>::type call_max_size(const Alloc& a)
{
    return a.max_size();
}
 size_type allocator_traits<Alloc>::max_size(Alloc& a) noexcept {
   return a.max_size(); //TODO: It may not be implemented!
 }