# File lib/edi4r.rb, line 544
    def add( obj, auto_validate=true )
      err_msg = "Added object must also be a "
      if obj.is_a? Message
        @content = :messages unless @content
        raise TypeError, err_msg+"'Message'" if @content != :messages
      elsif obj.is_a? MsgGroup
        @content = :groups unless @content
        raise TypeError, err_msg+"'MsgGroup'" if @content != :groups
      else
        raise TypeError, "Only Message or MsgGroup allowed here"
      end
      obj.validate if auto_validate
      super( obj )
    end