@Override
  public ExprNodeDesc clone() {
    ExprNodeColumnListDesc clone = new ExprNodeColumnListDesc();
    clone.columns.addAll(columns);
    return clone;
  }
Dummy desc only for populating TOK_ALLCOLREF and should not be used
outside of TypeCheckProcFactory
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeColumnListDesc.java