Add expander to CP-9 compressor?

[Lizard King]

I'm laying out a tone pad based CP-9 compressor for my next project.  I saw this note in an old thread about what to do with the unused half of the LM13600:

"Use an inverted copy of the envelope signal to drive the second OTA as an expander."

My old ART SGX 2000 paired an expander with a compressor to cut down on noise but I've never seen foot pedals that do that.  I'm not much of a designer so could anyone give me some pointers on actually doing something like this?

[PRR]

Inverting the compressor envelope will not make a noise-gate.

Maybe there's a plan for a 13600 noise-gate somewhere?

Or find any old noise-gate that works, and don't fret the "unused half" of the 13600. It's only 7 transistors. Your PC/Mac/iPad has over 700,000 transistors and most of the time most of them are "unused".