def extra=(extra)
      raise ArgumentError, "Extra must begin with \\305, \\306 or \\307" unless extra =~ /^[#{CODEA+CODEB+CODEC}]/n
      type, data = extra[0,1], extra[1..-1]
      @extra = class_for(type).new(data)
    end
def extra=(extra)
      raise ArgumentError, "Extra must begin with \\305, \\306 or \\307" unless extra =~ /^[#{CODEA+CODEB+CODEC}]/n
      type, data = extra[0,1], extra[1..-1]
      @extra = class_for(type).new(data)
    end
Set the extra for this barcode. The argument is a string starting with the
"change character set" symbol. The string may contain several character
sets, in which case the extra will itself have an extra.