one possible heuristic for being a guard

svn:r12427

doc/TODO

@@ -24,6 +24,12 @@ Items blocking 0.2.0.10-alpha:
     - Delay the separation of flags till 0.2.1.x. -NM
     - Let's come up with a good formula for Guard.
 
+Here's a go:
+
+  Take the 7/8s of the Running servers that have the oldest birthdates.
+  To be a guard, you must be in this set and you must at least match the
+  median WFU of the set.
+
   - Should we ship with a fallback-consensus? Where in the tarball does
     it go? What's the process for choosing it?
     - We can, but we don't have to now.  Stick it in place of the