code: def _create_matrix(self, document, dictionary):
          
          sentences = document.sentences
  
          words_count = len(dictionary)
          sentences_count = len(sentences)
          if words_count < sentences_count:
              message = (
                  "Number of words (%d) is lower than number of sentences (%d). "
                  "LSA algorithm may not work properly."
              )
              warn(message % (words_count, sentences_count))
  
          # create matrix |unique words|×|sentences| filled with zeroes
          matrix = numpy.zeros((words_count, sentences_count))
          for col, sentence in enumerate(sentences):
              for word in map(self.stem_word, sentence.words):
                  # only valid words is counted (not stop-words, ...)
                  if word in dictionary:
                      row = dictionary[word]
                      matrix[row, col] += 1
  
          return matrix
docs: Creates matrix of shape |unique words|×|sentences| where cells
          contains number of occurences of words (rows) in senteces (cols).