CLIPS
CLIPS is a rule-based programming language and expert system shell, primarily used for building knowledge-based systems, including decision support systems, diagnostics, and real-time monitoring applications.
Hello, world code example:
(printout t "Hello world!" crlf)
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.
Repositories on GitHub: 1,884