Clause 13.1 in the 1999 edition of the FIDIC Red, Yellow and Silver Books provides that variations may be initiated by the Engineer/Employer at any time prior to issuing the Taking-Over Certificate. This may be either by an instruction or a request to the Contractor to submit a proposal. The 1999 Books provide for the Contractor at any time to submit for approval proposals to the Engineer/Employer which, if implemented, will accelerate completion; reduce the cost to the Employer of executing, maintaining or operating the works. The Engineer or Employer should respond as soon as practicable after receiving the proposal with approval, disapproval or comments, and the Contractor should not delay any work while awaiting the response. Clause 13.6 deals with adjustments for changes in laws and, as in the 1999 editions, provides for the Contractor to be able to claim an extension of time and/or additional cost incurred as a result of such changes.