# File lib/net/dns/rr/classes.rb, line 70
        def initialize(cls)
          case cls
          when String
            # type in the form "A" or "NS"
            new_from_string(cls.upcase) 
          when Fixnum
            # type in numeric form
            new_from_num(cls) 
          when nil
            # default type, control with Classes.default=
            @str = Classes.invert[@@default] 
            @num = @@default
          else
            raise ClassArgumentError, "Wrong cls class: #{cls.class}"
          end
        end