old_contents: class Sdr(object):
      pass
  
  class AnalogSdr(Sdr):
      pass
  
  class DiscreteSdr(Sdr):
      pass
  
new_contents: """SDR records
  
  These are really bare right now - look at the SDR commands to see
  how they actually get filled out.
  """
  
  class Sdr(object):
      """Base SDR record type for others to inherit"""
      pass
  
  class AnalogSdr(Sdr):
      """An analog SDR record"""
      pass
  
  class DiscreteSdr(Sdr):
      """A discrete SDR record"""
      pass
  
subject: add docstrings