code: def add(self, name: str, default=None, description: str=None):
          
          if name in self._vars:
              raise ConfigError(f"{self.name}.{name} already defined.")
  
          if name == 'name':
              raise ConfigError("'name' is a reserved name for a group.")
  
          v = _Var(name, description=description, default=default)
          self._vars[name] = v
docs: Add a variable named |name| to this value group, optionally giving it a
          default value and a description.
  
          Variables must be added with this method before they can be set or read.
          Reading a variable replaces the variable that was defined previously, but
          updates the description if a new one is set.