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
        """
        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()