code: def by_key(self, style_key, style_value):
          
          if self.style_types[style_key] is bool:
              style_attr = style_key
          else:
              style_attr = style_value
  
          def proc(_, result):
              return self.render(style_attr, result)
          return proc
docs: Return a processor for a "simple" style value.
  
          Parameters
          ----------
          style_key : str
              A style key.
          style_value : bool or str
              A "simple" style value that is either a style attribute (str) and a
              boolean flag indicating to use the style attribute named by
              `style_key`.
  
          Returns
          -------
          A function.