code: def move_cursor_one_letter(self, letter=RIGHT):
          
          assert letter in (self.RIGHT, self.LEFT)
  
          if letter == self.RIGHT:
              self.cursor += 1
              if self.cursor > len(self.text):
                  self.cursor -= 1
          else:
              self.cursor -= 1
              if self.cursor < 0:
                  self.cursor += 1
docs: Move the cursor of one letter to the right (1) or the the left.