code: def join_next_line(self, separator=' '):
          
          if not self.document.on_last_line:
              self.cursor_position += self.document.get_end_of_line_position()
              self.delete()
  
              # Remove spaces.
              self.text = (self.document.text_before_cursor + separator +
                           self.document.text_after_cursor.lstrip(' '))
docs: Join the next line to the current one by deleting the line ending after
          the current line.