if ((lockToken != null || txId != null) && subscriptionId != null) {
throw new DavException(DavServletResponse.SC_PRECONDITION_FAILED, "Ambiguous headers: either TransactionId/Lock-Token or SubscriptionId can be present, not both.");
}
DavSession session = null;
// try to retrieve a cached session
if (lockToken != null && containsReference(lockToken)) {
session = getSessionByReference(lockToken);
} else if (txId != null && containsReference(txId)) {
session = getSessionByReference(txId);