(DAV:valid-sync-token): The DAV:sync-token element value MUST map
to a valid token previously returned by the server;
Depending on the meaning you give to the term "valid", an out of date
token can be seen as "a valid token previously returned by the server",
in which case this precondition does not apply to out of date token.
So we could either add a DAV:no-expired-sync-token precondition or
change the definition of the existing one to include tokens previously
returned by the server but which have expired.
Should we add a "Use with" in the precondition definition as in
(http://tools.ietf.org/html/rfc4918#section-16) and would it be 403 or
409 ? In any case, it would be worth mentioning that the client can
solve this precondition failure by initiating a full sync.