public int length()
    {
        int total = 0;
        for ( int i=0;i<rows.size();i++ )
        {
            Row r = rows.get( i );
            for ( int j=0;j<r.cells.size();j++ )
            {
                FragList fl = r.cells.get( j );
                for ( int k=0;k<fl.fragments.size();k++ )
                {
                    Atom a = fl.fragments.get( k );
                    total += a.length();
                }
            }
        }
        return total;
    }