大家好，想請教如下例句中，as在句首的詞性為何，是準關係代名詞還是介係詞呢?
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.
我自己的目前認為這句的AS是準關係代名詞，AS後面省略的be動詞，所以句子可以還原成
As is necessary to ensure such right,
請問這樣理解對嗎? 謝謝版上的高手。