code: def drop_fringe_punctuation(string_matrix: List[List[str]]) -> List[List[str]]:
      
      return [[drop_edge_punct(word)
               for word in sentence]
              for sentence in string_matrix]
docs: Drop punctuation that occurs on the edges of words.
      :param string_matrix: a data matrix: a list wrapping a list of strings, with each sublist being a sentence.
      >>> drop_fringe_punctuation([['this' , 'is', 'cool!'], ["'But", 'so', 'is', 'this'] ])
      [['this', 'is', 'cool'], ['But', 'so', 'is', 'this']]