r which client B may make a PUT request attempting to change
    the same document from version 1 to version 3.

    In this case, client B can add an &#39;If-Match: &#34;1&#34;&#39; header, which
    would trigger a 412 Conflict response code, since the current
    version (&#34;2&#34;) does not match the version required as a condition by
    the header If-Match header (&#34;1&#34;).

    Client B is now aware of the conflict, and may consult the user,
    saying the update to version 3 failed. The user m