Vintage Computing Christmas Challenge 2022

Author:   Peter De Wachter
Category: Christmas Challenge
System:   IBM 5110
Language: APL
Len source code: 30 characters
Len exe file:    N/A
Len code only:   N/A
Instructions:
   You can find a web-based emulator for the IBM 5110 here:
   https://norbertkehrer.github.io/ibm_5110/emu5110.html

   Documentation for this machine is available from bitsavers:
   http://www.bitsavers.org/pdf/ibm/5110/

   The emulator is configured to boot into BASIC by default. Use the
   switches to the right of the screen to restart in APL mode.
   
   When the 5110 is booted in APL, you can just type in the program,
   just like a BASIC system. Use the shift key to type APL
   characters. The character ⌽ requires an overstrike: shift-M, cursor
   left, shift-O.

   Unfortunately the 5110's screen has only 16 lines, so it's too
   small to show the entire Christmas star. If you have a real
   machine, you can send the output to the printer.

   The program will also work with modern APL implementations. Try
   running it on tryapl.org.

Description:
   The program is equivalent to this:

   V ← 4 3 2 1 ¯4 ¯3 ¯2 ¯1 0 ¯1 ¯2 ¯3 ¯4 1 2 3 4
   S ← V ∘.≤ -V
   ' *'[1+S]

   I found this by trial and error, starting from the standard idiom
   for creating a triangular matrix. There's probably a mathematical
   theory that explains why this works, but I don't know it :)