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given good (or asset), would the price be set to $p$. The equilibrium price $p^*$ is then such that $D(p^*) = S(p^*)$, which maximizes the amount of good exchanged among agents, given the set of preferences corresponding to the current supply and demand curves\cite{Solow_1956}. In reality, the full knowledge of $S(p)$ and $D(p)$ is problematic, and Walras envisioned his famous {\it t\^atonnement} process as a mean to observe the supply/demand curves. However, there is a whole aspect of the dynamics of markets that is totally absent in Walras' framework. While it describes how a pre-existing supply and demand would result in a clearing price, it does not tell  us anything about what happens \emph{after} the transaction has taken place. In this sense, the Walrasian price is of very limited scope, since the theory ceases to apply as soon as the price is discovered.  A practical solution to match supply and demand is the so-called ``order book'' \cite{harris1990liquidity,glosten1994electronic}, book''harris 1990\cite{harris1990liquidity,glosten1994electronic},  where each agent posts the quantities s/he is willing to buy or sell as a function of the price $p$. $S(p)$ (resp. $D(p)$) is then the sum of all sell (buy) quantities posted at or above (below) price $p$. At each time step, the auctioneer can then clear the market by finding the (unique) price such that $D(p^*) = S(p^*)$. This is   in fact how most financial markets worked before the advent of electronic matching engines, when market makers played the role of ``active'' Walrasian auctioneers, in the sense that they would themselves contribute to the order book as to insure orderly trading and stable prices \cite{glosten1985bid,madhavan2000market}. 
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