code: def distinct_letters(string_matrix: List[List[str]]) -> Set[str]:
      
      return set([letter
                  for sentence in string_matrix
                  for word in sentence
                  for letter in word])
docs: Diagnostic function
      :param string_matrix: a data matrix: a list wrapping a list of strings, with each sublist being a sentence.
      :return:
      >>> dl = distinct_letters([['the', 'quick', 'brown'],['how', 'now', 'cow']])
      >>> sorted(dl)
      ['b', 'c', 'e', 'h', 'i', 'k', 'n', 'o', 'q', 'r', 't', 'u', 'w']