code: def format(self, vertices):
          
          buf = io.StringIO()
  
          buf.write(self.name + '\n')
          buf.write('{\n')
          buf.write('    type {};\n'.format(self.type_))
          buf.write('    faces\n')
          buf.write('    (\n')
          for f in self.faces:
              s = f.format(vertices)
              buf.write('        {}\n'.format(s))
          buf.write('    );\n')
          buf.write('}')
          return buf.getvalue()
docs: Format instance to dump
          vertices is dict of name to Vertex