power. (4) When withdrawing obligations, the Authority may, by a reasoned decision, set the final date for their implementation. (5) After having conducted a repeated market analysis, the Authority may amend the imposed obligations by applying the necessary changes according to the provisions of this section concerning the imposition of obligations. Cost accounting obligation 51. (1) The Authority shall require operators having significant market power to establish cost accountting for the purposes of regulation. (2) Cost accounting must show separate accounts, in accordance with international best practices. (3) Costs relating to regulated and non-regulated activities are kept separate. (4) Cost accounting must be by activity-based costing. (5) The cost accounting system must be audited annually by an independent body appointed by the Authority, the costs of the audit to be borne by the operator having significant market power which allow the Authority to publish a cost nomenclature prior to submission of the reference interconnection offer for approval. (6) Pending the implementation of cost accounting, the interconnection rates shall be calculated on the basis of the following recommendations(a) using a regional benchmark; (b) using an existing cost calculation tool; (c) using, for an initial period of three years, a top-down model based on forward-looking historical costs, before moving to a model based on a long-run incremental costs, which gives the operator with a 72