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/// When included in a GET request, the Observe Option extends the GET
/// method so it does not only retrieve a current representation of the
/// target resource, but also requests the server to add or remove an
/// entry in the list of observers of the resource depending on the
/// option value. The list entry consists of the client endpoint and the
/// token specified by the client in the request. Possible values are:
///
/// `0` (register) adds the entry to the list, if not present;
///
/// `1` (deregister) removes the entry from the list, if present