code: def unique(iterables):
      
      included_elements = set()
  
      def included(element):
          result = element in included_elements
          included_elements.add(element)
          return result
      return [element for elements in iterables for element in elements
              if not included(element)]
docs: Create an iterable from the iterables that contains each element once.
  
      :return: an iterable over the iterables. Each element of the result
        appeared only once in the result. They are ordered by the first
        occurrence in the iterables.