n the message
                  payload, as well as once for the last chunk. The invocation happens
                  after the chunk header is available but before any body octets
                  have been parsed.
                </p>
                <p>
                  The extensions are provided in raw, validated form, use <a class="link" href="../ref/boost__beast__http__basic_chunk_extensions/parse.html" title="http::basic_chunk_extensions::parse"><code class="computeroutput"><span class="identifier