code: def declare(self, name, description=None, **kwargs):
      
      if not self._is_valid_key(name):
        raise self.InvalidKeyError(
            'Invalid key name, must begin with a lowercase letter', name)
      if name in self._declarations:
        raise self.KeyAlreadyDeclaredError(
            'Configuration key already declared', name)
      self._declarations[name] = self.Declaration(
          name, description=description, **kwargs)
docs: Declare a configuration key with the given name.
  
      Args:
        name: Configuration key to declare, must not have been already declared.
        description: If provided, use this as the description for this key.
        **kwargs: Other kwargs to pass to the Declaration, only default_value
            is currently supported.