code: def add(self, name: Any) -> None:
          
          self._check_keywords([str(name)])
          super().add(str(name))
docs: Before adding a new name, it is checked to be valid variable
          identifiers.
  
          >>> from hydpy.core.devicetools import Keywords
          >>> keywords = Keywords('first_keyword', 'second_keyword',
          ...                     'keyword_3', 'keyword_4',
          ...                     'keyboard')
          >>> keywords.add('1_test')   # doctest: +ELLIPSIS
          Traceback (most recent call last):
          ...
          ValueError: While trying to add the keyword `1_test` to device ?, \
  the following error occurred: The given name string `1_test` does not \
  define a valid variable identifier.  ...
  
          >>> keywords
          Keywords("first_keyword", "keyboard", "keyword_3", "keyword_4",
                   "second_keyword")
  
          After correcting the string, everything works fine:
  
          >>> keywords.add('one_test')
          >>> keywords
          Keywords("first_keyword", "keyboard", "keyword_3", "keyword_4",
                   "one_test", "second_keyword")