Solutions for Chapter 6

/cc @fabioyamate @laurocaetano
I didn't search for, but was thinking if there was a function for decrementing values.

Is there any reason for using `(pred n)` instead of `(n - 1)`?

In Clojure is also preferable to use `(inc n)` instead of `(+ n 1)`, it reads better, the `inc` but because it doesn't support infix notation.