 value is `boost::none` (unseated optional) and
            `ec` does not contain an error, this indicates the end of the
            body, no more buffers are present.

        @li If the optional contains a value, the first element of the
            pair represents a &lt;em&gt;ConstBufferSequence&lt;/em&gt; containing one or
            more octets of the body data. The second element indicates
            whether or not there are additional octets of body data.
            A value of `true` means there 