Net Book Value shall be calculated based upon depreciation on a straight
line basis over a 20 year period beginning on the purchase date, as set forth
in Appendix C. Under no circumstances shall Customer be obligated to
reimburse or otherwise keep ENA whole with respect to, or take assignment of,
any agreement or obligation entered into by ENA after the effective date of
this Agreement (or any amendment of any agreement existing prior to the
effective date of this Agreement) without Customer's prior written approval
of such agreement or obligation.