On 2018-06-04 20:12, Noam Postavsky wrote:
> I couldn't find any other cases of optional parameters in core macros.

I'm not sure which macros count as core macros.  In cl-lib itself there's cl-assert, which seems to behave the same as cl-incf.

In the rest of Emacs there are lots of other examples.  Many of them (for example semantic-find-tags-by-name or calendar-increment-month) seem to work when passed a nil-valued variable, but many others behave like cl-incf (for example gnus-summary-article-score).

Clément.