docs: def add_style(self, name, style_type, builtin=False):
          """
          Return a newly added style object of *style_type* and identified
          by *name*. A builtin style can be defined by passing True for the
          optional *builtin* argument.
          """
          style_name = BabelFish.ui2internal(name)
          if style_name in self:
              raise ValueError("document already contains style '%s'" % name)
          style = self._element.add_style_of_type(
              style_name, style_type, builtin
          )
          return StyleFactory(style)