zlacker

Here is toy example: I define a function f(x):=1+x2. J will automatically work out the inverse for me, as you can see below:
f=:(1+*&2) f 1 2 3 4 3 5 7 9 (f^:_1)f 1 2 3 4 1 2 3 4 (f^:_1) 1 2 3 4 0 0.5 1 1.5
Now obviously not every function is bijective or even if bijective, trivial to invert -- and J doesn't (or at least didn't) have a super well specified way of computing those generalized inverses. But still: "under" is actually pretty cool, even just conceptually I find it quite valuable.