docs: def neighbors(self):
          '''
          Returns the left and right neighbors as Word instance.
          If the word is the first one in the sentence only the right neighbor is returned and vice versa.
          '''
          if len(self._sentence) == 1:
              return {
                  'left': None,
                  'right': None
              }
          else:
              p = self._position
              if -1 < p < len(self._sentence):
                  if 0 == self._position:
                      return {
                          'left': None,
                          'right': self._sentence.word(p+1)
                      }
                  elif 0 < self._position < len(self._sentence) - 1:
                      return {
                          'left':  self._sentence.word(p-1),
                          'right': self._sentence.word(p+1)
                      }
                  else:
                      return {
                          'left': self._sentence.word(p-1),
                          'right': None
                      }
              else:
                  raise IndexError()