 If <span class="emphasis"><em>BufferSequence</em></span> meets the requirements of <span class="emphasis"><em>MutableBufferSequence</em></span>,
          then this type will be <code class="computeroutput"><span class="identifier">net</span><span class="special">::</span><span class="identifier">mutable_buffer</span></code>,
          otherwise this type will be <code class="computeroutput"><span class="identifier">net</span><span class="special">::</span><span class="identifier">const_buffer</span></code