docs: def left_margin(self, margin):
          '''Specify the left margin.
          
          Args:
              margin: The left margin, in character width. Must be less than the media's width.
          Returns:
              None
          Raises:
              RuntimeError: Invalid margin parameter.
          '''
          if margin <= 255 and margin >= 0:
              self.send(chr(27)+'I'+chr(margin))
          else:
              raise RuntimeError('Invalid margin parameter.')