docs: def transpose(self) -> 'TextDiagramDrawer':
          """Returns the same diagram, but mirrored across its diagonal."""
          out = TextDiagramDrawer()
          out.entries = {(y, x): _DiagramText(v.transposed_text, v.text)
                         for (x, y), v in self.entries.items()}
          out.vertical_lines = [_VerticalLine(*e)
                                for e in self.horizontal_lines]
          out.horizontal_lines = [_HorizontalLine(*e)
                                  for e in self.vertical_lines]
          out.vertical_padding = self.horizontal_padding.copy()
          out.horizontal_padding = self.vertical_padding.copy()
          return out