Actually it is possible...

Suppose there is a trading system which facilitates trading of three distinct kinds of asset: money, e-gold and e-gold-gen. (E-gold-gen is a weird e-gold derivative.)

There are separate order books for e-gold and for e-gold-gen, however, there is one peculiarity in this system: it can match e-gold  bids with e-gold-gen asks, and e-gold asks with e-gold-gen bids under some conditions.

As long as e-gold price is around target price, no cross-matching is done, i.e. e-gold bids are only matches with e-gold asks.

Now suppose e-gold price is below the target price. When person tries to sell e-gold below target price, his ask is matched with e-gold-gen bid.

If X is e-gold ask price and Y is e-gold-gen bid price, then one unit of e-gold creates X/Y units of e-gold-gen.

I.e. we reduce supply of e-gold and increase supply of e-gold-gen. As people sell more and more e-gold it eats through e-gold-gen orderbook, and more and more e-gold-gen is created. (Lower the price of e-gold-gen, more of it is created per unit of e-gold.)

Obviously, this works as long as there is a demand for e-gold-gen. If this is an economic abstraction, there are always people who want to buy at very low price, so it always works.

In reality, at some point people will stop buying e-gold-gen no matter what's the price, so whole system will collapse at that point.

(I.e. if you approach it with supply/demand curves it works, but if you assume that demand is finite it doesn't.)

Now if price of e-gold is above the target, it works in different direction: e-gold bids are matched with e-gold-gen asks, and again you need X/Y units of e-gold-gen to create one unit of gold. If there is a lot of extra demand, it will eat through e-gold-gen asks, price of e-gold-gen will get higher and higher, and one unit of e-gold-gen will create more units of e-gold.