Terry Latz would have known ~
So I channeled him, and he suggested the standard recipe for foaming cleaners: Dawn, water, and a small shot each of amonia & white vinegar.
It worked fine, but it was quite blue, and since the rug is blue, it was perfect.
Next, we went with Tide Cold Water, a strong brightening formulation for the obvious reason. This left the rug a little stiff, unlike Kirby's product which leaves the rug plush. Vacuuming brought the softness back, but not to the degree that Kirby's secret ingredient does.
Both solutions sudsed and foamed wonderfully. For the next batch of carpet liquor, I'll add a little liquid Calgon. The Kirby stuff is 30 bucks a gallon so I saved a tidy sum, avoided the blizzard, and half of the upstairs carpeting is clean and DONE! Let me know if something works better.
Terry always liked my two Kirbies. It was comforting and so pleasant to think of him all afternoon, as the holidays approach in earnest with all the chores he so loved to do.