原文:While we cannot predict what the future holds, we do know that the student
has to practice modeling by being given problems from many different
applications "as" is done in this book.
连接词是用来连接句子的，很明显的is done in this book不是一个句子(缺少主词)
但有两种可能，第一种是主词被省略；第二种是as是介系词。
请问各位大大，我这样分析对吗? 我想分析as词性的原因，是因为想要查字典，知道as在
这里代表什么意思。

do know [that] is done in this book[that]=to practice....透过有问题的案例来学习，就如书中给予的实例

as (it) is 应该是惯用语，省略了 it ,常见句型如：as is done/ as is usually the case 等等表示“如(这般这般)”, 也只有表示“如”时能省略，不是所有 as it is 的 it 都能省略！参考下面剑桥例句：(conj) as (like)As is often the case with children,...https://goo.gl/btvkSd