Actually I spoke too soon. Putting

(setq org-html-mathjax-template (concat org-html-mathjax-template "
<script type=\"text/x-mathjax-config\">
MathJax.Hub.Register.StartupHook(\"TeX Jax Ready\",function () {
  MathJax.Hub.Insert(MathJax.InputJax.TeX.Definitions.macros,{
    cancel: [\"Extension\",\"cancel\"],
    bcancel: [\"Extension\",\"cancel\"],
    xcancel: [\"Extension\",\"cancel\"],
    cancelto: [\"Extension\",\"cancel\"]
  });
});
</script>
"))
​
in my init didn't fly when I rebooted, although it seemed to work before
when I stuck it in my init and did eval-buffer. Going back to

#+begin_html
<script type="text/x-mathjax-config">
 MathJax.Hub.Config({
   TeX: {extensions: ["cancel.js"]},
   tex2jax: {
     inlineMath: [ ['$','$'], ['\\(','\\)'] ],
     processEscapes: true
   }
 });
</script>
#+end_html

Would appreciate any advice on how to incorporate the \cancel code
into org-html-mathjax-template.