OTHER CLASSI FICATIONS. A contract consisting of mutual exec utory promises is said to be "bilateral," as in the exec utory contract of sale, where one is bound to buy and pay the price, and the other to deliver the thing sold. Where the consideration is executed on one side and executory on the other, the contract is said to be "uni lateral," as where an option is given to buy or sell. (Leake on Contracts, 18.) A contract is said to be aleatory or hazardous when the performance of its object depends on an uncertain event, as in the contract of insurance. A contract is certain when the thing to be done depends on the will of the party, or when it must happen in the usual course of events. See Louisiana Code, Secs. 1766-76.