09年新教材166页最上面的一段话怎么理解~~

“ where a chattel on which capital allowances have been obtained is sold at a loss, the allowance cost for chargeable gains purposes is reduced by the lower of the loss and the net amount of allowances given (taking into account any balance allowances or charges). the result is no gain and no loss."

能不能用具体例子说明一下，谢谢~~