It's another toy - an implementation of a "Rational" UDT including operators and indexing.
However it's written in Java, not C, and therefore may be much more accessible to a newbie since it doesn't require same level of familiarity with the server internals as a C extension. The drawback is that it has limitations due to the need to go through a java/C layer - but it should be enough for many extension developers.
(Performance shouldn't be an issue with any modern JVM.)
To my horror I just realized I never made the source available on github. Eek!
At some point I'll probably update the articles and republish them on Medium. E.g., this discusses integration with Hibernate but now I would add jOOQ.
Anyway I'm sure several of the pl/x extensions could also be used to write extensions, esp. if they're just UTF extensions, so it might be helpful to include a section in the FAQ. My article covers a lot of the SQL-side stuff that took a while to figure out. It's all obvious and clearly documented - once you know what to look for. But before then...
Bear