return toString( "" );
    String toString( String prefix )    {        StringBuffer buff = new StringBuffer();        buff.append( prefix + "Stress collection (event="                     + getEventName() + "):\n" );        Iterator iter = _stress_set.iterator();        while( iter.hasNext() )        {            StressInstance stress = (StressInstance) iter.next();                        buff.append( stress.toString( prefix + "\t" ) + "\n" );        } // while iter        return buff.toString();            } // method toString