If the covenants are concurrent, one party can not be required to perform while the other party has not yet performed and is not able and willing to perform.1 The purchaser can not be compelled to pay before the seller has delivered substantially all of the goods which he has agreed to deliver.2 If a building contract provides for payment at a certain date upon completion, the owner can not be compelled to pay on such date if the contractor has not yet completed the performance of his covenants.3 If the covenants are concurrent, the fact that one of the parties is unable to perform before the adversary party is able and willing to perform, and offers performance on his part, docs not of itself discharge the contract.4