from job in session.Linq
( ) where officeIds.Contains( job.Office.Id ) select job;
The book Linq in Action, as well as Matt Warren's series on query providers, both provided a lot of help in understanding the code. As I started sifting through it and looking at failing tests, I started making some fairly sweeping changes to the code. Not necessarily to check it in, mind you, more for my own eddy-ficashun. I was in spike mode, not "someone else may actually look at this" mode.
In the end, it wasn't quite as hard as I first thought. 'Course, I have a built-in advantage since parsing expression trees is much like parsing the family tree, what with all the AndAlso's and OrElse's. ExclusiveOr was a new concept though.
By the time I was done, I had an ExpressionVisitor very similar to the one Matt Warren specified and I had implemented it in a couple of the visitors in the codebase. It still didn't do what I wanted but I was a lot clearer on how things flowed. It fixed a couple of bugs while introducing others and while I wasn't any further ahead in my specific task, it was a nice break to do something I haven't in a long time.
Then I updated to the latest and lo! Chadly (who I'd link to if I had a blog for him) has implemented what appears to be the same thing, albeit in a much cleaner way. And even better! Doug Mayer (also conspicuously unlinked) has implemented the exact Contains support that I needed! Well, almost. Contains works on arrays now but not on Lists (or anything deriving from ICollection methinks). But ToArray helps out there.
Anyway, here I am a day later having learned about query providers and my work still got done for me in my absence. All hail OSS!
Kyle the NHiberLinq'd