Yep, my indespensibleness is that I have all this mostly useness knowledge in my head, and if you show me the problem, I can usually get something useful out of my brain to get somewhere quickly. And if not, I can skim the code and get an idea; and if not, I can sort of operate a debugger.

What do you want me to write down to help someone else do that? Read all the things, just in case!