请老师看一下这句话中as是作什么成分
Even before the coronavirus hit, trouble was brewing in the land of unicorns, as tech startups worth more than $1bn are called.
as tech startups worth more than $1bn are called 为比较分句，对unicorns作补充说明。《剑桥语法》将这种补充说明性的成分剖析为补充语supplement。你可把它剖析为传统语法的插入语（说话人对句子或句子中某个词项的讲解）。