This was pretty much trial and error. I knew I had to insert a
¯ (negative number) and use mathematical functions like
○ (trigonometic) or
⍟ (logarithm) that yield complex number for negative input, or directly insert a
J (imaginary part).
After a lot or errors, I finally tried inserting a
7 and a
Try it online in ngn/apl demo, the online interpreter recommended by the cop.
Note that this won't work in, e.g., Dyalog APL, which requires a space before the high minus.