# File lib/ai4r/data/data_set.rb, line 196
      def << data_item
        if data_item.nil? || !data_item.is_a?(Enumerable) || data_item.empty?
          raise ArgumentError, "Data must not be an non empty array."
        elsif @data_items.empty?
          set_data_items([data_item])
        elsif data_item.length != num_attributes
          raise ArgumentError, "Number of attributes do not match. " +
                  "#{data_item.length} attributes provided, " +
                  "#{num_attributes} attributes expected."
        else
          @data_items << data_item
        end
      end