I made a fairly substantial number of changes, expanding the introduction and background information and rewrote the whole parsing section. I won't claim that it's finished but at least you will have something to talk about :-)
Also, I managed to upload it to the folder twice and I haven't figured out how to get rid of the extra copy.
Anyway, let me know if you think it is sufficiently precise and readable (there's a fine line...) Also, I have tried to keep things slightly VOMS-agnostic as we may have other ACs "for free"