prevailing in similar cases, but such claim may not be made in advance of the use and it shall not be enforceable after the lapse of two years from the time when the owner of copyright work first received knowledge of the circumstances which give rise to the claim: Provided that the owner of the copyright may not claim to have received such knowledge after the expiration of six years from the date of the contract to be amended. (5) Where the user does not exercise an exclusive right conferred on him by the owner of copyright the owner may revoke such right if the non-exercise thereof was prejudicial to his legitimate interests. (6) The right of revocation under subsection (5) may be exercised only after the period of delay stipulated in the contract for commencing the exercise of the exclusive right by the person on whom it is granted has expired but, in any case, not earlier than two years: (a) from the grant of such right; or (b) if the work to be used was supplied subsequent to the grant of the right, from the date of its delivery. (7) In each of the cases under paragraph (a) or (b) of subsection (6) the owner of copyright shall notify the user of the proposed revocation, allowing him a reasonable additional time in which to exercise the exclusive right, but where the exercise of the right by the user has become or is impossible or the user has refused the right, the owner's right of revocation may be exercised in accordance with subsection (6). (8) A contract on future grant of rights for the use of works to be created thereafter and not specified in detail but only mentioned in general or by reference to their nature, may be terminated by either party with six months notice after a period of four years from the conclusion of the contract. Commissioned work 16. (1) Where a contract is in respect of work to be created (in this Act referred to as "commissioned work") the user shall be under an obligation to make a declaration respecting acceptance of the work within sixty days from the date on which the work was handed over or, if such work is regulated under another written law, within such period as is prescribed for acceptance under that law, and if the user fails to make such declaration within such period he shall be deemed to have accepted the work. (2) Within the time allowed under subsection (1) for acceptance of commissioned work, the user may, once or repeatedly, return the work to the author and require him to amend or correct the work taking into 20