[Checkers] Number of checkers

[Michael Ernst]

I would like to say, throughout the paper, that we have 5 checkers.  Saying
that we have 4+1 feels more clumsy, and I don't think claiming 5 is
inaccurate.  I don't want to make this change while Matt is editing section
3.  Matt, could you make the change throughout the paper?  (Search for
"four" and "4".)

                     Thanks,

                    -Mike