template <typename Beg, typename End, typename Sought>
auto find_prefix(const Beg beg,
                 const End end,
                 Sought sought)
{
  auto low = std::lower_bound(beg, end, sought);
  ++sought.back();
  auto up = std::lower_bound(beg, end, sought);
  return std::make_tuple(low, up);
}