Experienced Common Lisp professionally for the first time. An experience difficult to forget.
Working in an AI (nlp) platform extracting structured info from unstructured text.
Appart from working on the maintenance of one of the main tools, I started from scratch a real time api with a pubub model where subscribers connect to an endpoint adding a custom filter in the querystring, and automatically creating (or assigning if already existed) a channel with live news matching the filters.
Counting of the number of news that arrived to a given user was a tricky one to do it efficiently.