public void addVersions( BitSet bs )
    {
        this.versions.or( bs );
        for ( int i=0;i<cells.size();i++ )
        {
            FragList fl = cells.get(i);
            for ( int j=0;j<fl.fragments.size();j++ )
            {
                Atom a = fl.fragments.get(j);
                if ( a instanceof Fragment )
                {
                    Fragment f = (Fragment) a;
                    if ( f.isEmpty() )
                        f.versions.or(bs);
                }
            }
        }
    }