NilClass, TrueClass, FalseClass special cased for singleton classes?

If you do:

class << nil
   self
end

it returns NilClass - this is the case for true and false as well.

It appears that this behavior is special-cased for at least these three wel=
l-known singleton objects. Two questions:


1)      Are there others?

2)      Why is this the case - it seems like there was effort to make this =
happen vs. no effort to do what Ruby does with 'normal' singleton classes.

Thanks,
-John