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 been enacted by the Congress of the United States of America before March 31, 1979.
 (6) After the date of entry into force of the Treaty, the Panama Canal Commission shall, unless otherwise provided by legislation enacted by the Congress of the United States of America, be obligated to reimburse the Treasury of the United States of America, as nearly as possible, for the interest cost of the funds or other assets directly invested in the Commission by the Government of the United States of America and for the interest cost of the funds or other assets directly invested in the predecessor Panama Canal Company by the Government of the United States of America and not reimbursed before the date of entry into force of the Treaty. Such reimbursement for such interest costs shall be made at a rate determined by the Secretary of the Treasury of the United States of America and an annual intervals to the extent earned, and if not earned, shall be made from subsequent earnings. For purposes of this reservation, the phrase "funds or other assets directly invested" shall have the same meaning as the phrase "net direct investment" has under section 62 of title 2 of the Canal Zone Code.
(b) UNDERSTANDINGS:
 (1) Before the first day of the three-year period beginning on the date of entry into force of the Treaty and before each threeyear period following thereafter, the two*Parties shall agree upon the specific levels and quality of services, as are referred to in paragraph 5 of Article III of the Treaty, to be provided during the following three-year period and, except for the first three-year period, on the reimbursement to be made for the costs of such services, such services to be limited to such as are essential to the effective functioning of the Canal operating areas and the housing areas referred to in paragraph 5 of Article III. If payments made under paragraph 5 of Article III for the preceding three-year period, including the initial three-year period, exceed or areless than the actual costs to the Republic of Panama for supplying, during such period, the specific levels and quality of services agreed upon, then the Panama Canal Commission shall deduct from or add to the payment required to be made to the Republic of Panama for each of the following three years one-third of such excess or deficit, as the case may be. There shall be an independent and binding audit, conducted by an auditor mutually selected by both Parties, of any costs of services disputed by the two Parties pursuant to the reexamination of such costs provided for in this understanding.
 (2) Nothing in paragraph 3, 4, or 5 of Article IV of the Treaty may be construed to limit either the provisions of the first paragraph of Article IV providing that each Party shall act, in accordance with its constitutional processes, to meet danger threatening the security of the Panama Canal, or the provisions of paragraph 2 of Article IV providing that the United States of America shall have primary responsibility to protect and defend the Canal for the duration of the Treaty.