Use preconditions and postconditions in the implementation

```
These are new features in Clojure 1.1.
We can use preconditions to check the input parameters and postconditions 
for validation instead of custom macros/functions
Guide: http://clojure.org/special_forms
```

Original issue reported on code.google.com by `draga...@gmail.com` on 17 Dec 2009 at 10:12
