大家好，想请教如下例句中，as在句首的词性为何，是准关系代名词还是介系词呢?
As necessary to ensure such rights, the exclusionary rule, which prohibited
the introduction into evidence of material seized in violation of the Fourth
Amendment, likewise applied to the State's prosecution of state crimes.
我自己的目前认为这句的AS是准关系代名词，AS后面省略的be动词，所以句子可以还原成
As is necessary to ensure such right,
请问这样理解对吗? 谢谢版上的高手。