public String toString()
    {
        Stack<Step> stack = new Stack<Step>();
        Step temp = this;
        while ( temp != null )
        {
            stack.push( temp );
            temp = temp.parent;
        }
        StringBuilder sb = new StringBuilder();
        while ( !stack.isEmpty() )
        {
            Step s = stack.pop();
            if ( s.kind != FragKind.empty )
                sb.append("x="+s.x+" kind="+s.kind.toString()+"\n" );
        }
        return sb.toString();
    }