@Override
 public String toString() {
   StringBuffer bf = new StringBuffer();
   bf.append(sourceQuery.toString());
   bf.append(" ==> ");
   bf.append(targetQuery.toString());
   return bf.toString();
 }
 @Override
 public OBDARDBMappingAxiom clone() {
   OBDARDBMappingAxiom clone =
       new RDBMSMappingAxiomImpl(this.getId(), sourceQuery.clone(), targetQuery.clone());
   return clone;
 }