I feel like I flubbed an interview because, while I was describing my work with xpaths, the interviewer caught me off guard by asking how I would implement a simple xpath engine. I was caught off guard and answered "I think that's a mistake. I've seen a lot of half-assed xpath implementations. I would rely on the engine optimized by very smart people, as if it were a compiler." Really the interviewer was asking "Do you know about trees and graphs?"