code: def add_entry(entry)
        raise IOError, 'non-writable archive' unless writable?
        raise IOError, 'closed archive' if closed?
        unless entry.kind_of?(Entry) then
          raise ArgumentError, 'Archive::Zip::Entry instance required'
        end
  
        @entries << entry
        self
      end
docs: Adds _entry_ into a writable ZIP archive.
  
   <b>NOTE:</b> No attempt is made to prevent adding multiple entries with
   the same archive path.
  
   Raises Archive::Zip::IOError if called on a non-writable archive or after
   the archive is closed.