presto-main/src/main/java/io/prestosql/sql/planner/iterative/GroupReference.java/GroupReference/GroupReference
class GroupReference:    public GroupReference(PlanNodeId id, int groupId, List<Symbol> outputs)
    {
        super(id);
        this.groupId = groupId;
        this.outputs = ImmutableList.copyOf(outputs);
    }