code: def merge_next(letters: List[str], positions: List[int]) -> List[str]:
      
      for pos in positions:
          letters[pos] = letters[pos] + letters[pos + 1]
          letters[pos + 1] = ""
      return letters
docs: Given a list of letter positions, merge each letter with its next neighbor.
  
      :param letters:
      :param positions:
      :return:
  
      >>> merge_next(['a', 'b', 'o', 'v', 'o' ], [0, 2])
      ['ab', '', 'ov', '', 'o']
      >>> # Note: because it operates on the original list passed in, the effect is not cummulative:
      >>> merge_next(['a', 'b', 'o', 'v', 'o' ], [0, 2, 3])
      ['ab', '', 'ov', 'o', '']