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)
        com = ' '.join(self.vnames)  # for comment
        return '({0:s})  // {1:s} ({2:s})'.format(index, self.name, com)