Return a |Length| object specifying the width of available "writing"
        space between the margins of the last section of this document.

def _block_width(self):
        """
        Return a |Length| object specifying the width of available "writing"
        space between the margins of the last section of this document.
        """
        section = self.sections[-1]
        return Emu(
            section.page_width - section.left_margin - section.right_margin
        )