请问下面这句英文怎么理解?
since you make delivery before Christmas a firm condition,we deeply regret that we cannot supply you as we have done on so many occasions in the past. 特别是最后半句as we have done on so many occasions in the past.该如何翻译，谢谢．
since you make delivery before Christmas a firm condition,we deeply regret that we cannot supply you as we have done on so many occasions in the past. 既然你使在圣诞节前交货成为定局，那么我们深感抱歉，我们不能像我们以往很多场合下那样给你供货。 to have done 表示的是肯定动作,不是 not supply.
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