docs: def _merge(cls, default, user):
          """Append user options to default options. Return yala section."""
          section = cls._CFG_SECTION
          merged = default[section]
  
          if section not in user:
              return merged
  
          user = user[section]
          for key, value in user.items():
              if key in merged:
                  merged[key] += ' ' + value
              else:
                  merged[key] = value
          return merged