main.protected is not defined
main (self at toplevel) has public and private, but doesn't have protected.
protected #=> NoMethodError
It looks a bit weird.
I guess it was forgotten to add main.protected when Module#protected was introduced
in Ruby 1.1b9_05.
main.protected seems not to be useful because it makes methods of Object protected, but
it might be good to have main.protected for consistency.