Русско-англо-русский юридический словарь

The narrower form of the exception is based on the idea that state A may not complain of the non-performance of an obligation owed to it by state B where such performance has been prevented by the prior wrongful act of state A. (Australian Year Book of International Law)

A non-performance of an obligation is fundamental if strict compliance with the obligation which has not been performed is of essence to the contract; ... (Unidroit)

In case of nonperformance of an obligation to transfer an individually-defined thing to ownership, to economic management, to operative administration or to compensated use to a creditor, the latter has the right to demand the taking of this thing from the debtor and its transfer to the creditor on the conditions provided by the obligation. (The Civil Code of the Russian Federation translated by Peter B. Maggs with A.N. Zhiltsov)