std::size_t file_handle::write_some_at(uint64_t offset,
                                       const ConstBufferSequence& buffers)
{
  error_code ec;
  std::size_t s = write_some_at(offset, buffers, ec);
  detail::throw_error(ec);
  return s;
}
 size_t write_some(implementation_type& impl,
     const ConstBufferSequence& buffers, boost::system::error_code& ec)
 {
   return write_some_at(impl, 0, buffers, ec);
 }