Thrush is the "T combinator" - I believe that the "Thrush" name comes from To Mock a Mockingbird by Raymond Smullyan [1].

[1]: https://www.amazon.com/Mock-Mockingbird-Other-Logic-Puzzles/...

[2]: https://en.wikipedia.org/wiki/Combinatory_logic#In_computing

[3]: https://leanpub.com/combinators/read#leanpub-auto-the-thrush