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
        return 'arc {0:s} ({1.x:18.15g} {1.y:18.15g} {1.z:18.15g}) '\
                '// {2:s} ({3:s})'.format(
                        index, self.interVertex, self.name, vcom)