It's in several, particularly newer APL dialects; see https://aplwiki.com/wiki/Under#History https://aplwiki.com/wiki/Under#History . Proud to say I originated

It's in several, particularly newer APL dialects; see https://aplwiki.com/wiki/Under#History https://aplwiki.com/wiki/Under#History . Proud to say I originated the "structural" form used by Uiua, which is able to deal with transformations like filtering that lose parts of the input. Every language now seems to have its own take on what exactly Under means, with different implementations leading to different supported functions and various ways to relax the theory to be more convenient or handle intuitively expected things better.