Cool language. I happened to notice the ⍜ operator, which operates on a transformed array, then reverts the transformation. Not sure if other array languages include this, but it's a really cool idea. I always found the traditional map/filter operators to be limiting in this regard, kind of like trying to write expressions without using parentheses.
It's in several, particularly newer APL dialects; see 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.
