@Override
    public Builder end() {
      ExpressionTree current = currentTree.removeFirst();
      if (current.getChildren().size() == 0) {
        throw new IllegalArgumentException("Can't create expression " + root +
            " with no children.");
      }
      if (current.getOperator() == ExpressionTree.Operator.NOT &&
          current.getChildren().size() != 1) {
        throw new IllegalArgumentException("Can't create not expression " +
            current + " with more than 1 child.");
      }
      return this;
    }
Generate the backwards compatible string for test cases
@return the sarg using the old string
storage-api/src/java/org/apache/hadoop/hive/ql/io/sarg/SearchArgumentImpl.java