Cybervein add two types of new smart contracts, the contract units, into normal DAG. The contract unit contains signature, transaction, parent hash, lineal parent and timestamp. The lineal parent  refers to the last transaction of the smart contract, which guarantees  all users who use the contract are always in the same state (even if it has short-term furcation and ensure the bifurcation finally), it could guarantee the atomicity and consistency of contract.