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"/>        

             </productionFrontier>