Updated docs

CONTRIBUTING.MD

@@ -28,4 +28,9 @@ What to do
  - Tag merge commit on `master` (`git tag -a v1.0.0 -m "Version 1.0.0"`)
  - `cargo publish`
  - Update web demo & pull server (`make docker`)
- - Update aur package
+ - Update aur package
+
+Test `default.nix` with
+``sh
+nix-build -E 'let pkgs = import <nixpkgs> { }; in pkgs.callPackage ./default.nix {}'
+``

TODO.md

@@ -20,6 +20,8 @@
  - Sums and products with functional arguments
  - Add functions: gcd, inverse mod, dice
  - Tuple operations
+ - Number theory: select a group, inverses, etc
+ - Negative mod
 
 ## Prompt
  - Fix terminal color detection