| defaultLabel | If no exeuction frames are present this becomes the default alias to be resolved. | 
| expr | Default expression to look to resolve the alias. If null, use the runtime to resolve the alias. | 
string Returns a resolved alias.
Try to resolve an alias value using the SearchExpressionRuntime attached to this context. Each frame if asked to resolve a SearchExpression.alias.
[Description("Returns ids of current selection")]
[SearchExpressionEvaluator(SearchExpressionEvaluationHints.ThreadNotSupported)]
public static IEnumerable<SearchItem> SelectionIds(SearchExpressionContext c)
{
    var instanceIds = UnityEditor.Selection.instanceIDs;
    foreach (var id in instanceIds)
    {
        yield return SearchExpression.CreateItem(id, c.ResolveAlias("selected id"));
    }
}