code: def add_keyword(self, keyword, schema=None, source=None):
          
          keyword_dict = self._sourced_dict(source, value=keyword)
  
          if schema is not None:
              keyword_dict['schema'] = schema
  
          self._append_to('keywords', keyword_dict)
docs: Add a keyword.
  
          Args:
              keyword(str): keyword to add.
              schema(str): schema to which the keyword belongs.
              source(str): source for the keyword.