docs: def stringPropertyNames(self):
          r"""
          Returns a `set` of all keys in the `Properties` object and its
          `defaults` (and its `defaults`\ ’s `defaults`, etc.)
  
          :rtype: `set` of text strings
          """
          names = set(self.data)
          if self.defaults is not None:
              names.update(self.defaults.stringPropertyNames())
          return names