It's currently integrating higher-order logic (HOL) with a reimplementation of HOL Light.
(update) their doesn't appear to be any FM. Can anyone post any links to papers/lists/etc.?
(edit) I still don't see any rationale for the theorem prover kernel re-implementation. Again, pointers appreciated.
(I work part time on the Brain team, but I haven't asked the authors about this -- this is a best guess based upon knowing TF, not an informed statement.)
[1] https://www.quora.com/Which-programming-languages-does-Googl...