A collection of valid production rules. You could have a generic production frontier for all players, or specify individual rules. See productionRule

             <productionFrontier name="production">
                     <frontierRules name="buyInfantry"/>
                     <frontierRules name="buyArtillery"/>
                     <frontierRules name="buyArmour"/>
                     <frontierRules name="buyFighter"/>
                     <frontierRules name="buyBomber"/>
                     <frontierRules name="buyTransport"/>
                     <frontierRules name="buySubmarine"/>
                     <frontierRules name="buyCarrier"/>
                     <frontierRules name="buyDestroyer"/>
                     <frontierRules name="buyBattleship"/>
                     <frontierRules name="buyAAGun"/>
                     <frontierRules name="buyFactory"/>        

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