private String getMemoizedSubString(int start, int end) {
    return  (astStr == null || start < 0 || end > astStr.length() || start >= end) ? null :
      astStr.subSequence(start, end).toString();
  }
Tag this ASTNode with information about the object from which this node
originated.
parser/src/java/org/apache/hadoop/hive/ql/parse/ASTNode.java