@Override
  public String toString() {
    return "UNDEFINED";
  }
An UNDEFINED value is the result of accessing an attribute of a null-valued attribute. If you
access an attribute that has an explicit value of null, then it is not undefined. For example, if
a query accesses the attribute address.city and address is null, then the result is undefined. If
the query accesses address, then the result is not undefined, it is null.

@version $Revision: 1.1 $
geode-core/src/main/java/org/apache/geode/cache/query/internal/Undefined.java