We study a simple exchange model in which price is fixed and the amount of a
good transferred between actors depends only on the actors' respective budgets
and the existence of a link between transacting actors. The model induces a
simply-connected but possibly multi-component bipartite graph. A trading
session on a fixed graph consists of a sequence of exchanges between connected
buyers and sellers until no more exchanges are possible. We deem a trading
session "feasible" if all of the buyers satisfy their respective demands.