prototype.sol is an account management model of DeltaChain. In the account management model every contract has a master account, which, by default, is the creator of a contract. The master account plays the role of a contract manager, which has some authorities including "setMaster", "setCreditPlan", "addUser", "removeUser" and "selectSponsor". Every contract keeps a list of users who can call the contract for free but limited by credit. The user of a specific contract can be either added or removed by the contract master. Although from a user's perspective the fee is free, it is paid by a sponsor of the contract. Anyone can be a sponsors of a contract, just by calling sponsor function, and also the sponsor identity can be cancelled by calling unsponsor function. A contract may have more than one sponsors, but only the current sponsor chosen by master need to pay the fee for the contract. If the current sponsor is out of energy, master can select sponsors from other sponsors candidates by calling selectSponsor function. The creditPlan can be set by the master which includes credit and recoveryRate. Every user have the same original credit. Every Transaction consumes some amount of credit which is equal to the fee of the Transaction, and the user can also pay the fee by itself if the gas payer is out of the credit. The credit can be recovered based on recoveryRate (per block).