code: def format(self, vertices):
          
          index = ' '.join(str(vertices[vn].index) for vn in self.vnames)
          vcom = ' '.join(self.vnames)  # for comment
          buf = io.StringIO()
  
          buf.write('spline {0:s}                      '\
                  '// {1:s} ({2:s})'.format(
                          index,self.name, vcom))
          buf.write('\n     (\n')
          for p in self.points:
              buf.write('         '+p.format()+'\n')
          buf.write('\n     )\n')
          buf.write('')
          return buf.getvalue()
docs: Format instance to dump
          vertices is dict of name to Vertex