def backspace(self):
        """
        Move cursor back one character. Do not cross lines.
        """
        cursor_xs = self.cursor_xs - 1
        try:
            if self._buffer[cursor_xs] != '\n':
                self.cursor_x = cursor_xs
        except IndexError:
            self.cursor_x = 0

        Move cursor back one character. Do not cross lines.
        
system/shscreens.py