cument or folder&#39;s current
    version. A PUT request MAY have an &#39;If-None-Match: *&#39; header [<a href="#ref-COND">COND</a>],
    in which case it MUST fail with a 412 response code if the document
    already exists.

    In all &#39;ETag&#39;, &#39;If-Match&#39; and &#39;If-None-Match&#39; headers, revision
    strings should appear inside double quotes (&#34;).

    A provider MAY offer version rollback functionality to its users,
    but this specification does not define the user interface fo