# File lib_wpm/wpm.rb, line 1756
    def check_mapping
      [ [ 'list',  @list,  true  ],
        [ 'item',  @item,  true  ],
        [ 'index', @index, false ]
      ].each do |prop_name, prop_value, required|
        case (prop_value)
        when NilClass
          if (required) then
            raise PropertyError, "required `#{prop_name}' property at #{widget_id.inspect}"
          else
            # nothing to do.
          end
        when Proc, Method
          # nothing to do.
        else
          raise PropertyError, "invlid #{prop_name} property type at #{widget_id.inspect}"
        end
      end

      nil
    end