1 line
(I.2= /”1 n){[n=.3 3#:i.*:3

3 lines
cp=. 3 3 #: i. *: 3
ix=. I. 2= /”1 cp
ix { cp

0 2
1 1
2 0

for boxes
<”1 ix { cp
|0 2|1 1|2 0|

(zf (* x y)
(x ‘(1 2 3 4))
(y ‘(1 2 3 4))
if (evenp x)
(oddp y))

I used zf for the name, as list comprehensions are sometimes called Zermelo–Fraenkel expressions, and because zf is really short and stands out. The if marker in the middle breaks the sets from the conditions, avoiding the prefix <- form and makes the whole thing slightly easier on the eyes. (Again, I’m not a Lisper, so I don’t have the Paren-Sight yet ;-)

It’s cool to see someone else doing the same thing.