code: def list_available(cls) -> List[str]:
          
          keys = list(Registrable._registry[cls].keys())
          default = cls.default_implementation
  
          if default is None:
              return keys
          elif default not in keys:
              message = "Default implementation %s is not registered" % default
              raise ConfigurationError(message)
          else:
              return [default] + [k for k in keys if k != default]
docs: List default first if it exists