@Beta
    public static TestAndSetCondition ofRequiredTimestampWithSelectionFallback(long requiredTimestamp, String selectionFallback) {
        return new TestAndSetCondition(requiredTimestamp, selectionFallback);
    }
<p>Returns a TestAndSetCondition with a required timestamp, but which has a transparent
fallback to evaluating a selection string if the backend does not support timestamp conditions.
This has the following semantics:</p>
<ul>
    <li>If the backend supports test and set conditions with timestamp requirements,
        the condition passes iff the currently persisted timestamp exactly matches
        the timestamp in the condition. The selection condition string is completely
        <em>ignored</em> in this case.</li>
    <li>If the backend is too old to support timestamp requirements the condition
        selection string is evaluated as if there was <em>no</em> required timestamp
        present.</li>
</ul>

<p><strong>Note:</strong> this API is currently in Beta.</p>

@param requiredTimestamp required timestamp, or 0 for no timestamp requirement.
@param selectionFallback condition selection string that will be evaluated by nodes
                         that are on a version too old to support timestamp requirements.
document/src/main/java/com/yahoo/document/TestAndSetCondition.java