docs: def get_knob_defaults(cls):
          """ Returns a string with defaults
          >>> Knob.get_knob_defaults()
          '# \n# HAVE_RUM=True\n\n# Yar\n# JOLLY_ROGER_PIRATES=124\n\n# Foo Bar\n# WUNDER=BAR\n'
          """
  
          return '\n'.join(
              [
                  '# {description}\n# {knob}={default}\n'.format(
                      description=cls.get_registered_knob(name).description,
                      knob=name,
                      default=cls.get_registered_knob(name).default
                  ) for name in sorted(cls._register.keys())
              ]
          )