To Do List
----------

- Add heterogeneous equality back into the kernel.
- Finish Sigma-type support.
- Build Nat and List theories.
- Fix usability issues identified when formalizing optional and sum types.
- Add unification hints support in the Lean front-end.
- Improve simp tactic interface (more configuration options).
- Add ssreflect-like rewrite commands.
- Add record-type (as syntax sugar for Sigma-types).
- Implement inductive datatypes and recursive function package from first principles. The goal is to use the HOL/Isabelle approach.
- Generic Tableaux prover.
- [MCSat](http://leodemoura.github.io/files/fmcad2013.pdf) framework.
- Independent type checker using a different programming language (e.g., F* or OCaml).
- Module for reading [OpenTheory](http://www.gilith.com/research/opentheory/) proofs.
- Re-implement apply-tactic.