code: def is_visible(self, x, y):
          
          return ((x >= 0) and
                  (x <= self.width) and
                  (y >= self._start_line) and
                  (y < self._start_line + self.height))
docs: Return whether the specified location is on the visible screen.
  
          :param x: The column (x coord) for the location to check.
          :param y: The line (y coord) for the location to check.