[Checkers] Todos in section 3

[Michael Ernst]

I have added some todo items for you in section 3, mostly asking why
certain features need to be implemented procedurally rather than
declaratively.  You can search for your name, but not all of the todos are
prefixed by a name.  If you could add some notes in response (or point me
at elsewhere in the paper where just that issue is discussed), that would
be helpful.

I'm going to turn in.  I have edits on paper that are ready to apply to
sections 3.7-4.2, so if you could refrain from editing those for now,
except to address todos, I would appreciate it.  But I need to sleep before
my morning train, I haven't started packing yet, etc.

                     Thanks,

                    -Mike