e breaking opportunity between two "x"
 * There is a line breaking opportunity between two "あ"
 * There is a line breaking opportunity after "x" or "あ"
   unless suppressed by the next character (BA, GL).
 * There is a line breaking opportunity before "x" or "あ"
   unless suppressed by the previous character (BB, GL).
-->
<section class=ref>
  <div>xx&#x2006;<br>あ<br>あ&#x2006;<br>あ<br>あ&#x2006;<br>xx</div><!-- BA, tailorable -->
</section>
<section>
  <div>xx&#x2006;ああ&#x2006;ああ&#x2006