old_contents: 
  class IdError(Exception): pass
  class NullError(IdError): pass
  class CheckDigitError(IdError): pass
  class LengthError(IdError): pass
  class CountryCodeError(IdError): pass
  class CharacterError(IdError): pass
  class CheckSumError(IdError): pass
new_contents: 
  class IdError(Exception): pass
  
  class NullError(IdError): pass
  class CharacterError(IdError): pass
  class CheckDigitError(IdError): pass
  class CheckSumError(IdError): pass
  class CountryCodeError(IdError): pass
  class LengthError(IdError): pass
  class NullError(IdError): pass
subject: order subclass IdErrors alphabetically.