Hi!

What°«s the best way of checking if an argument has been given or not?

That is, what°«s the best way of achieving

class A
  ArgumentMissing = Object.new.freeze

  def b(c = ArgumentMissing)
    if ArgumentMissing.equal? c
      puts 'no argument was given'
    else
      puts c
    end
  end
end

A.new.b # => no argument was given
A.new.b 1 # => 1

?  I remember seeing something that didn°«t require a constant like
this, but I can°«t seem to find it.  (Note: I don°«t want to use nil for
this particular case.  I would preferably use a value that is
inaccessible outside of A.)