[O] TODO list sorted across categories

[Edward DeMeulle]

I'd like to be able to produce a TODO list that is sorted by timestamp
independently of category. I'm still new to elisp and so don't yet
understand all the code involved. What I've been able to do so far is
create a separate version of org-todo-list by changing a single line:

      (org-set-sorting-strategy 'timestamp-up) ; was 'todo

however that appears to sort by timestamp within categories, not
across. Is there a relatively simply way to have org-todo-list ignore
categories?