Well done! I had a somewhat similar experience twenty years ago in the informatics olympiads. I had no idea about the whole world of algorithms, complexity, etc. until a friend recommended to do the first round of our national informatics olympiad. The first round was pretty simple programming, the second round was a whole new world.
By the way "Ng" is Andrew Ng's surname, it's not NG with a capital G.
Btw a visa is a document allowing entry into a country, while VISA is a word mark used by Visa, inc. for their payment cards and network. I think you're referring to the travel document, but since the context also includes payment networks, I'm not 100% sure.
The point GP is making, is that everyday mathematics is not done in Lean, but in a language that's more like JavaScript, where equals doesn't always mean equals, + and • mean whatever seems convenient at the time, and objects sometimes change type without notice.
The AAP says: "infants younger than 1 year are developmentally unable to learn the complex movements, such as breathing, necessary to swim. They may manifest reflexive swimming movement under the water but cannot effectively raise their heads to breathe. There is no evidence to suggest that infant swimming programs for those younger than 1 year are beneficial."