Festive Lattice

Author:          Diminished
Category:        Christmas Challenge
System:          BBC Microcomputer System (Model B, OS 1.2)
Language:        BASIC (BBC BASIC II)
Len source code: 71
Len exe file:    64
Len code only:   64

Instructions:
The easiest emulator to use is the browser-based jsbeeb:

https://bbcmicro.co.uk/jsbeeb/

Using jsbeeb, select "Discs" from the menu bar, then "From examples or local", and choose the "disc.ssd" disc image file included with this ZIP. Then, either press Shift + F12 to autoboot the disc, or alternatively type CHAIN "PROGRAM", and press Enter. Note that on the BBC Micro, the double-quote character " is typed by pressing Shift + 2.

(Here are some other viable emulators:)
b-em:               http://b-em.bbcmicro.com/
beebjit:            https://github.com/scarybeasts/beebjit/
BeebEm for Windows: http://www.mkw.me.uk/beebem/
b2:                 https://github.com/tom-seddon/b2

Description:
Basic BASIC. Just draws parallel diagonal lines using a pair of loops -- one parallel set with positive gradient, the other parallel set with negative gradient. Then it uses the BBC Micro's text-windowing facility (plus CLS) to clear away excess mess.