Add section on accountability

README.org

@@ -1342,6 +1342,54 @@ now both refuse to forward messages.
 
 **** Accountability
 
+Both Ben and Lem contact Alyssa and insist neither of them made those
+edits to the document.
+Couldn't they please get access again to write to the file?
+
+That evening, Alyssa thinks about it and decides that yes, she could,
+if next time she could hold whoever did it accountable so she could
+prevent the problem from happening again and know who violated her
+trust.
+
+Alyssa makes two new capabilities, but these ones are a little bit
+different than before: while both allow writing to the file, this time
+she associates each one with the name of the person she is handing it
+out to.
+Now if Bob writes to the file, it's logged that Bob made this change,
+and if Lem writes to the file, it's logged that Lem made this change.
+Alyssa hands out these new write-capable-but-logging ocaps to Bob and
+Lem and logs off for the evening.
+
+The next morning, the file is defaced again.
+But the logger picks it up: Lem made all these changes!
+Alyssa revokes the capability she gave to Lem and gives him a call
+on the phone.
+
+Lem swears, he really didn't make these changes!
+Alyssa shows him her proof, and Lem thinks about it.
+Well... Lem is really sure that he didn't make those changes, but
+he knows that Mallet wanted access to the file.
+It could be that Mallet asked him for it when they went out
+drinking and Lem was intoxicated... or it could be that Mallet used
+that opportunity to insert a backdoor into his device.
+Lem really isn't sure, but insists that /he/ is not the one that did
+it.
+
+Alyssa trusts Lem enough as a person (but not as a person who
+practices good security hygeine), and distrusts Mallet enough, that
+she finds this story plausible.
+Still she considers with satisfaction that placing the blame "on the
+capability she gave to Lem", whether or not it was Lem that did it,
+was what she really needed to get to the bottom of the situation.
+"For now, you can email me suggestions," Alyssa tells Lem.
+"But the next time you want to collaborate on a document, make sure
+you're more careful with your authority.
+And if you're not sure whether Mallet might have a backdoor in your
+system or not, maybe it's time to do a thorough exorcism of your
+computer."
+Lem apologizes and agrees... he plans to try to audit his computer
+tonight.
+
 **** Composition
 
 # add backup of file example; alice's composed capability should