Tests for binders with multiple arguments.

These are expected to fail for now.
Fix #15 Parse binders with multiple arguments
Fixed a bug that causes type-checking arithmetic expressions to loop
Merge pull request #46 from plmday/master

Fix #45.
Improved debug information
Merge pull request #47 from plmday/master

Reword debug information for type checking.

See discussion in #45.
Add examples about basic arithmetics (see #21).
Systematically renamed `Nat' to `Int' and `nat' to `int' (fix #49)
Clean up multiple argument binders implementation.
Merge pull request #52 from morphling/mult-args-binders

Parse binders with multiple arguments
Merge pull request #51 from plmday/master

Rename Nat to Int.

The built-in type Nat always supported negative values,
so the clarify things, it is better to call it Int.
Format comment for haddock.

Try `cabal haddock` to see the difference.