关于本章

本章描述 extended visitor pattern，这是在阅读 eli.thegreenplace The Expression Problem and its solutions 时所发现的，在 ./The-Expression-Problem-and-its-solutions 中收录了这篇文章。

除此之外，以下文章中也描述了 extended visitor pattern:

rice.edu Understanding the Extended Visitor Pattern

neu.edu Functional Visitors Revisited

ed.ac.uk Independently Extensible Solutions to the Expression Problem

stackoverflow Extend Visitor pattern without modifying the original source code?