def define_group(name, range)
      define_method(name) do
        children[range]
      end
      private(name)
      memoize(name)
    end
def define_group(name, range)
      define_method(name) do
        children[range]
      end
      private(name)
      memoize(name)
    end
Define a group of children

 @param [Symbol] name
 @param [Range] range

 @return [undefined]

 @pai private