code: def format_boundary_section(self):
          
          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()
docs: format boundary section.
          assign_vertexid() should be called before this method, because
          vertices reffered by faces should have valid index.