private GrouperDefOrAggregatorDef(GrouperDef <T> grouper, AggregatorDef <T> aggregator)
 {
     Grouper    = grouper;
     Aggregator = aggregator;
 }
 public static GrouperDefOrAggregatorDef <T> Create(AggregatorDef <T> aggregator)
 {
     return(new GrouperDefOrAggregatorDef <T>(null, aggregator));
 }