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

def format_boundary_section(self):
        """format boundary section.
        assign_vertexid() should be called before this method, because
        vertices reffered by faces should have valid index.
        """
        buf = io.StringIO()
        buf.write('boundary\n')
        buf.write('(\n')
        for b in self.boundaries.values():
            # format Boundary instance and add indent
            indent = ' ' * 4
            s = b.format(self.vertices).replace('\n', '\n'+indent)
            buf.write(indent + s + '\n')
        buf.write(');')
        return buf.getvalue()