> I would have said it would either throw an error, concatenate the vectors or only add the matching elements Heh this itself is common evidence of an (experi

> I would have said it would either throw an error, concatenate the vectors or only add the matching elements Heh this itself is common evidence of an (experienced) programmer's way of thinking. Remember, when dealing with mathematics, there's no machine (or runtime or similar abstraction) there lurking in the background, enforcing conditions, or even lending a physical reality. Operations in mathematics are defined. Notation in mathematics is just an operation encoded as an "arbitrary" symbol. Which leads to > I just don’t think it’s particularly clear, and I struggle to believe it’s clear, unambiguous and obvious to statisticians too. This happens to be convention in both a subset of textbooks and many programming environments. It's mostly an artifact of the notation. If you want to explore what math notation "feels like" a bit more without learning math (which I _wholeheartedly_ recommend as it's incredibly useful), try out the APL programming language a bit. It evokes a similar atmosphere of notation conveying the idea of well-defined operations.