code: def format_vertices_section(self):
          
          buf = io.StringIO()
          buf.write('vertices\n')
          buf.write('(\n')
          for v in self.valid_vertices:
              buf.write('    ' + v.format() + '\n')
          buf.write(');')
          return buf.getvalue()
docs: format vertices section.
          assign_vertexid() should be called before this method, because
          self.valid_vetices should be available and member self.valid_vertices
          should have valid index.