We discovered that an intertemporal economy that has spot market trading in each period will have the same allocation as the Arrow-Debreu equilibrium as long as there is one good that can be traded forward into all periods. [We also need to assume that consumers correctly forecasti/expect the spot market prices in each period.] Provide the intuition for this result. How come only spot market trading is not the same, but spot market trading with one good also traded forward is enough to replicate the allocation when all goods are 'traded at once' ?