Dynamic Ligatures Stacking Glyphs using open type

I have inherited a font project where ligatures are dynamically built/stacked as you type. I’ve searched extensively for any info about how this can be made in Glyphs but found nothing, I could be using the wrong terms?

Any help on how to do this would be amazing.

Here’s a demo of how it works.

What do you type to make the letters smaller?

A§ =2/3 size
A§§ = 1/2 size
A§§§ = 1/3 size

I don’t have to original working file and i’m not sure if it was made in Glyphs. This is the code i can see in Glyphs:

script DFLT;
language dflt;
lookup SUB_0 {
	sub A section section section by A.3;
	sub A section section by A.2;
	sub A section by A.1;
} SUB_0;
lookup SUB_0;

This is the first part of the game. Then you need to add a second lookup that moves the prices on top of each other. There are several options. This depends on what glyphs you have in your file. Either you build all kind of ligature glyphs (like A_A.1_3, A_A.2_2, A_A.3_1. Or you have each glyphs twice, one on the baseline and one hanging from the cap hight. Then use kerning to move them on top of each other.

There are only 4 size instances in the font and no other ligatures as stacks. The font in illustrator will automatically stack each size when they fit. I’m not sure I would like to create every ligature instance of all the short stacking glyph options, that would be way too many.

In the code, what does “SUB_0” relate to? I can’t find anything about this online.

SUB_0 is just the name of the lookup. Doesn’t mean anything.

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