Format instance to dump
        vertices is dict of name to Vertex

def format(self, vertices):
        """Format instance to dump
        vertices is dict of name to Vertex
        """
        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()