format vertices section.
        assign_vertexid() should be called before this method, because
        self.valid_vetices should be available and member self.valid_vertices
        should have valid index.

def format_vertices_section(self):
        """format vertices section.
        assign_vertexid() should be called before this method, because
        self.valid_vetices should be available and member self.valid_vertices
        should have valid index.
        """
        buf = io.StringIO()
        buf.write('vertices\n')
        buf.write('(\n')
        for v in self.valid_vertices:
            buf.write('    ' + v.format() + '\n')
        buf.write(');')
        return buf.getvalue()