private PredicateLeaf addLeaf(PredicateLeaf leaf) {
      PredicateLeaf result = leaves.get(leaf);
      if (result == null) {
        leaves.put(leaf, leaf);
        return leaf;
      } else {
        return result;
      }
    }
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