doc(doc/todo): update TODO list

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

doc/todo.md

@@ -1,20 +1,9 @@
 To Do List
 ----------
 
-- ~~Add `cast : Pi (A B : Type) (H : A = B) (a : A) : B`, and implement "casting propagation" in the type checker.~~
-- ~~Modify the `let` construct to store the type of the value. The idea is to allow `let name : type := value in expr`. The type does not need to be provided by the user. However, the user may want to provide it as a hint for the Lean elaborator.~~
-- ~~Refactor the elaborator code. The elaborator will be one of the main data-structures in Lean. The elaborator manager should be shared between different frontends.~~
-- ~~Reconsider whether meta-variables should be in `expr` or not. Metavariables (aka holes) are fundamental in our [design](design.md).~~
-- ~~Improve Lean pretty printer for Pi's. For example, it produces `Var a : Pi (A : Type) (_ : A), A` instead of `Var a : Pi (A : Type), A -> A`.~~
-- ~~Decide what will be the main technique for customizing Lean's behavior. The elaborator manager will have many building blocks that can be put together in many different ways. Possible solutions:~~
-   - ~~We design our own configuration language.~~
-   - ~~We use an off-the-shelf embedded language such as [Lua](http://www.lua.org).~~
-   - ~~We use Lean itself.~~
-- Module for reading [OpenTheory](http://www.gilith.com/research/opentheory/) proofs.
-- ~~Higher-Order unification and matching~~.
-- Rewriter (and Rewriter Combinators).
+- Add Sigma-types.
+- Define natural numbers, lists, inductive datatypes from first principles. The goal is to use the HOL/Isabelle approach.
+- Generic Tableaux prover.
 - [MCSat](http://leodemoura.github.io/files/fmcad2013.pdf) framework.
-- Implement independent type checker using a different programming language (e.g., OCaml).
-- Add `match` construct in `expr`.
-- Add inductive datatype (families) in the environment. It will be a new kind of object.
-- Add recursive function definition objects (low priority). It is another new kind of object.
+- Independent type checker using a different programming language (e.g., F* or OCaml).
+- Module for reading [OpenTheory](http://www.gilith.com/research/opentheory/) proofs.