老师，下面这个问题真的快要把我折磨死了，求解答。

句子：No doubt, had I taken a more deductive look at their lives, I would have envied them less. I didn't see, then, what a prison a house could be, since houses seemed to me brighter, handsomer places than any factory.

上述句子中的“what a prison a house could be”是什么语法知识呢？我初步判断是【省略it is的感叹句作表语，且表语前置】，但是我极其不确定这个答案，还恳请老师能够指点一下。