大家好，請教一下：
在Practical English grammar 3rd 177.11說：
Subject pronouns with forms of "be" can be left out after certain
conjunctions, especially in a formal style.
例句：
Start when ready. (= ... when you are ready.)
Phone me if (it is) necessary.
I'm enclosing my cheque for ￡50, as agreed.
而在518.1則說：
Common expressions with "as" in place of a subject: as follows; as was
expected; as was agreed; as is well known.
例句：
I am sending you the bill, as was agreed. (NOT ... as it was agreed.)