code: def unique_by_index(sequence):
      
      uniques = []
      for element in sequence:
          if element not in uniques:
              uniques.append(element)
      return uniques
docs: unique elements in `sequence` in the order in which they occur
  
      Parameters
      ----------
      sequence : iterable
  
      Returns
      -------
      uniques : list
          unique elements of sequence, ordered by the order in which the element
          occurs in `sequence`