(b) Concurrent, or
250. In the case of a condition subsequent, the rights of the promisee are determinable upon a specified event. The condition does not affect their commencement, but its occurrence brings them to a conclusion.
251. In the case of a condition concurrent, the promisee's rights are dependent upon his doing, or being ready to do, something simultaneously with the performance by the promisor.
252. In the case of a condition precedent, the promisee's rights do not arise until something has been done or has happened, or some period of time has elapsed.
253. Where the promise in a contract is conditional, the promisor may be discharged -
(a) By the promisee's failure to perform a concurrent condition.
(b) By the fact that there has been a total or substantial failure on the promisee's part to do that which he was bound to do under the contract - a state of things sometimes described as virtual failure of consideration.
(c) By the untruth of some one statement, or the breach of some one term, which the parties considered to be vital to the contract.