format edges section.
        assign_vertexid() should be called before this method, because
        vertices reffered by blocks should have valid index.

def format_edges_section(self):
        """format edges section.
        assign_vertexid() should be called before this method, because
        vertices reffered by blocks should have valid index.
        """
        buf = io.StringIO()
        buf.write('edges\n')
        buf.write('(\n')
        for e in self.edges.values():
            buf.write('  ' + e.format(self.vertices) + '\n')
        buf.write(');')
        return buf.getvalue()