- When adding a single element to a list, use .append() instead of
  += and creating a new list.
- For consistency, import the copy module, instead of just deepcopy. This
  leaves only a module at the class level, instead of a function.
- Improve some comments.
- Improve some whitespace.
- Use tuples instead of lists.
- Simplify a test.
(cherry picked from commit f96ddad)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>