[YorkshireSeason]

One should also mention Isabelle/HOL [1] and Concrete Semantics [2]. As of May 2018, Isabelle/HOL has the best proof automation, in case you want to get things done.

[1] https://isabelle.in.tum.de/

[unboxed_type]

True. It is worth pointing out that Isabelle/HOL does not have dependent types in its underlying logic making framework building a (much?) harder work.

[YorkshireSeason]

I'm not sure what you mean by "framework building" in this context.

[YorkshireSeason]

Whoops, forgot to link to _Concrete Semantics_.

[2] http://www.concrete-semantics.org