Add/remove an element into/from a cons.

[Hongyi Zhao]

How can I add/remove an element into/from a cons, taking the following
one as an example:


```emacs-lisp
(setq company-backends '((company-tabnine :separate company-capf
company-dabbrev company-keywords company-files company-ispell)))
```
How can I  remove `company-ispell` from and re-add it into the
company-backends defined above?

Regards,
HZ